Compounds, their preparation, related compositions, catalysts, electrochemical cells, fuel cells, and uses thereof

ABSTRACT

In some embodiments, this application relates to inventive compounds (e.g., Formula (I), Formula (II), thiosemicarbazones and/or thiosemicarbazones and their metal (e.g., zinc, cobalt, nickel, or copper) complexes, and extended structures thereof), methods for preparation of the inventive compounds, compositions comprising the inventive compounds (e.g., anode, cathodes, catalysts (e.g., electrocatalysts), glassy carbon electrodes, carbon paste electrodes, covalently modified carbon (e.g., modified graphene)), electrochemical cells comprising compositions that comprise one or more inventive compounds, fuel cells comprising compositions that comprise one or more inventive compounds, uses of one or more inventive compounds to produce H2 (e.g., via an electrochemical cell), and uses of one or more inventive compounds to create energy from H2 (e.g., via a fuel cell). Additional embodiments of the invention are also discussed herein.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part of International Application No. PCT/US2017/036815 filed Jun. 9, 2017, entitled “COMPOUNDS, RELATED COMPOSITIONS, CATALYSTS, ELECTROCHEMICAL CELLS, FUEL CELLS, THEIR PREPARATION AND THEIR USES” which is herein incorporated by reference in its entirety, and which claims the benefit of (1) U.S. Provisional Application No. 62/348,420, filed Jun. 10, 2016, entitled “COMPOUNDS, THEIR PREPARATION, RELATED COMPOSITIONS, CATALYSTS, ELECTROCHEMICAL CELLS, FUEL CELLS, AND USES THEREOF” which is herein incorporated by reference in its entirety and (2) U.S. Provisional Application No. 62/436,490, filed Dec. 20, 2016, entitled “COMPOUNDS, THEIR PREPARATION, RELATED COMPOSITIONS, CATALYSTS, ELECTROCHEMICAL CELLS, FUEL CELLS, AND USES THEREOF” which is herein incorporated by reference in its entirety.

This application claims the benefit of U.S. Provisional Application No. 62/719,972, filed Aug. 20, 2018, entitled “COMPOUNDS, THEIR PREPARATION, RELATED COMPOSITIONS, CATALYSTS, ELECTROCHEMICAL CELLS, FUEL CELLS, AND USES THEREOF, which is herein incorporated by reference in its entirety.

GOVERNMENT RIGHTS

This invention was made with government support under CHE-1361728 and CHE-1800245, both awarded by the National Science Foundation. The U.S. Government has certain rights in the invention.

BACKGROUND

Rising energy demands, coupled with growing concerns of repercussion from global climate change, have ignited considerable interest in the development of carbon neutral energy systems. Hydrogen is a promising component of these systems representing a light weight, energy dense energy carrier. Hydrogen evolution reactions (HERs), which involve a two-electron reduction of protons, can be used to store energy in H₂, with subsequent energy release through hydrogen oxidation reactions (HORs). Platinum is a catalyst for HER and HOR, yet its scarcity and high costs limit practical large scale application.

In addition, there remains a broader need for catalysts, related compounds, related compositions, and related electrochemical cells that have more desirable properties compared than currently exist, such as but not limited to: better efficiency (e.g., lower overpotential and higher turnover frequency), less expensive to produce, more easily synthesized, and more robust.

Certain embodiments of the invention can address one or more of the deficiencies discussed above.

In some embodiments, this application relates to inventive compounds (e.g., Formula (I), Formula (II), thiosemicarbazones and/or thiosemicarbazones and their metal (e.g., zinc, cobalt, or copper) complexes, and extended structures thereof), methods for preparation of the inventive compounds, compositions comprising the inventive compounds (e.g., anode, cathodes, catalysts (e.g., electrocatalysts), glassy carbon electrodes, carbon paste electrodes, covalently modified carbon (e.g., modified graphene)), electrochemical cells comprising compositions that comprise one or more inventive compounds, fuel cells comprising compositions that comprise one or more inventive compounds, uses of one or more inventive compounds to produce H₂ (e.g., via an electrochemical cell), and uses of one or more inventive compounds to create energy from H₂ (e.g., via a fuel cell). Additional embodiments of the invention are also discussed herein.

SUMMARY

Some embodiments of the present invention include a compound selected from Formula (I),

and

salts, optical isomers, geometric isomers, salts of isomers, and derivatives thereof.

Some embodiments of the present invention include a compound selected from Formula (II), M.L (II) and

salts, optical isomers, geometric isomers, salts of isomers, and derivatives thereof.

Yet other embodiments include a catalyst (e.g., an electrocatalyst) comprising a composition comprising a compound of Formula (I), a compound of Formula (II), or both. Some embodiments encompass an anode comprising a composition comprising a compound of Formula (I), a compound of Formula (II), or both. Other embodiments include a cathode comprising a composition comprising a compound of Formula (I), a compound of Formula (II), or both.

Still other embodiments of the invention include an electrochemical cell comprising a composition comprising a compound of Formula (I), a compound of Formula (II), or both. Additional embodiments include a fuel cell comprising a composition comprising a compound of Formula (I), a compound of Formula (II), or both.

Some embodiments of the invention include a method for producing H₂ comprising contacting, in an electrochemical cell, a first composition comprising a compound of Formula (I), a compound of Formula (II), or both with a second composition comprising water. Additional embodiments include a method for producing electricity comprising contacting, in a fuel cell, a first composition comprising a compound of Formula (I), a compound of Formula (II), or both with a second composition comprising H₂.

Further embodiments encompass a method for preparing a compound of Formula (I) comprising any suitable method, such as those disclosed herein, or a method for preparing a compound of Formula (II) comprising any suitable method, such as those disclosed herein. Additional embodiments encompass a method for preparing a catalyst (e.g., an electrocatalyst) comprising a composition comprising a compound of Formula (I), a compound of Formula (II), or both, comprising any suitable method, such as those disclosed herein. Still other embodiments include a method for preparing an anode comprising a composition comprising a compound of Formula (I), a compound of Formula (II), or both, comprising any suitable method, including those disclosed herein. Still further embodiments include a method for preparing a cathode comprising a composition comprising a compound of Formula (I), a compound of Formula (II), or both, comprising any suitable method, such as those disclosed herein.

Other embodiments of the invention are also disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the description of specific embodiments presented herein.

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

FIG. 1. Electrocatalytic H₂ evolution and H₂ oxidation. (A) Cyclic voltammograms of 3 mM ZnL in methanol with (from bottom to top) no added acid, 6 mM CH₃COOH, 9 mM CH₃COOH, and 12 mM CH₃COOH. Data collected at a scan rate of 0.5 V/s in the presence of 0.1 M Bu₄NPF₆ as supporting electrolyte. (B) Plot of icat versus [CH₃COOH] for 3 mM ZnL (upper lines at 3 mM CH₃COOH) at scan rates of 0.2 (Δ), 0.3 (◯), 0.4 (□), and 0.5 (⋄) V/s and 3 mM H₂L (lower lines at 3 mM CH₃COOH) at scan rates of 0.2(Δ), 0.5 (⋄), and 1.0 (×) V/s. (C) Cyclic voltammograms of 0.3 mM ZnL in methanol under 1 atm. H₂ (from top to bottom at 1.00 V) with no added base, 3 mM (CH₃CH₂)₃N, 6 mM (CH₃CH₂)₃N, 12 mM (CH₃CH₂)₃N, 21 mM (CH₃CH₂)₃N, and 30 mM (CH₃CH₂)₃N. Data collected at a scan rate of 0.5 V/s in the presence of 0.1 M Bu₄NPF₆ as supporting electrolyte. (D) Plot of i_(cat)/i_(p) versus [(CH₃CH₂N)₃] for 0.3 mM ZnL under 1 atm. H₂ (clustered lines second from the top) and 3 mM H₂L under 1 atm. H₂ (top line and bottom two lines) at scan rates of 0.2(Δ), 0.5 (⋄), and 1.0 (×) V/s.

FIG. 2. Mechanistic Studies of H₂ evolution. (A) Plot of charge versus time recorded during bulk electrolysis of 0.1 mM ZnL and 12 mM CH₃COOH in methanol with 0.1 M Bu₄NPF₆ as supporting electrolyte. (B) Comparisons of experimental (solid) and simulated (dotted) cyclic voltammograms for 3 mM ZnL and 12 mM CH₃COOH in methanol with 0.1 M Bu₄NPF₆ as supporting electrolyte at scan rates of (from bottom to top) 0.3, 0.4, and 0.5 V/s. (C) Concurrent catalytic pathways for hydrogen evolution through homo-coupling of neutral Zn(HL.) radicals and hetero-coupling of a neutral Zn(HL.) and cationic [Zn(H₂L.)]⁺ radicals.

FIG. 3. Energy profile along with spin densities of species involved in catalyzed H₂ evolution. Spin-density profiles for Zn(HL.) (A), and [Zn(H₂L.)]⁺ (B). (C) Relative energies (ZPE corrected) for H₂ evolution through the hetero-coupling of Zn(HL.) and [Zn(H₂L.)]⁺ using the B97-D/6-311G(d) level of theory. (D) Structure of the singlet [Zn₂(H₃L.₂)]⁺ transition state through the hetero-coupling pathway. See FIG. 20 for further information regarding the HER mechanism, analysis of the eigenvector associated with the imaginary frequency i1572 cm⁻¹, and the charge densities of atoms for H₂ evolution with respect to intrinsic reaction coordinate (IRC).

FIG. 4. ZnL in acetonitrile CV, v=0.2 V/s—0.1 M Bu₄NPF₆ acetonitrile solution with 3 mM ZnL vs Fc⁺/Fc⁰.

FIG. 5. ZnL HER acid concentration dependent CVs, v=0.2 V/s—0.1 M Bu₄NPF₆ acetonitrile solution with increasing concentrations of acid; vs Fc⁺/Fc⁰.

FIG. 6. H₂L HER acid dependent CVs, v=0.5 V/s—0.1 M Bu₄NPF₆ methanol solution with increasing concentrations of acid; vs Fc⁺/Fc (lowest concentration at bottom).

FIG. 7. H₂L HOR base concentration dependent CVs, v=0.5 V/s—H₂L HOR, H₂ atmosphere with increasing [Et₃N], vs Fc⁺/Fc⁰ (highest concentration of base at bottom).

FIG. 8. GC-TCD Readout—GC readouts of gas sampled every 30 minutes during 2.5 hour electrolysis of ZnL.

FIG. 9. ZnL Diffusion Limited CV Overlay—0.1 M Bu₄NPF₆ methanol solution with 3 mM ZnL run from v=0.1-1.0 V/s vs Ag/AgCl (highest mV/s at bottom).

FIG. 10. Peak currents vs square root of scan rate—0.1 M Bu₄NPF₆ methanol solutions with 3 mM ZnL, plot showing peak current plotted against the square root of the scan rate from diffusion limited CVs.

FIG. 11. ZnL concentration dependent CVs, v=0.5 V/s—0.1 M Bu₄NPF₆ methanol solution with 12 mM acetic acid added with increasing ZnL concentrations; vs Ag/AgCl (highest ZnL concentration at bottom).

FIG. 12. Plot of [ZnL]² vs catalytic current, v=0.5 V/s.

FIG. 13. Simulation of experimental data using DigiElch—ZnL HER CV Simulations of experimental data; 12 mM [acid]; v=0.2-0.5 V/s vs Fc⁺/Fc⁰ (lowest V/s at bottom).

FIG. 14. Simulation of experimental data using DigiElch—ZnL HER CV Simulations of experimental data; 12 mM [acid]; v=0.6-5.0 V/s vs Fc⁺/Fc⁰ (lowest V/s at bottom).

FIG. 15. Simulation of experimental data using DigiElch—ZnL HER CV Simulations of experimental data; 6 mM [acid]; v=0.2-0.5 V/s vs Fc⁺/Fc⁰ (lowest V/s at bottom).

FIG. 16. Optimized structure of [ZnHL]⁺ with protonation on hydrazino nitrogen.

FIG. 17. Optimized structure of [ZnHL]⁺ with protonation on sulfur.

FIG. 18. Optimized structure of [ZnHL]⁺ with protonation on amine nitrogen.

FIG. 19. Optimized structure of [ZnHL]⁺ with protonation on zinc.

FIG. 20. Transition state analysis of [Zn₂H₃L.₂]⁺—(Upper) Transition state geometry of [Zn₂H₃L.₂]⁺ along the HER pathway, shown with active N—H bond lengths associated with the imaginary frequency i1572 cm⁻¹, and equilibrium bond lengths in parentheses. (Lower) Charge densities of atoms near H₂ evolution with respect to IRC; N—H of [Zn(HL.)]⁺ (bottom and top curves), and N—H of Zn(H₂L.) (curves 2^(nd) from top and 2^(nd) from bottom).

FIG. 21. UV-Vis spectra from Spectroelectrochemical Electrolysis—(Upper) UV spectra recorded every 15 minutes during the electrolysis of 1 mM ZnL under applied potential of −1.7 V in 0.1 M Bu₄NPF₆ methanol solution. (Lower) Blow up of 350-500 nm region showing isosbestic point at 400 nm.

FIG. 22. CV after ZnL electrolysis—CV of ZnL after electrolysis in 0.1 M Bu₄NPF₆ methanol solution with 12 mM acetic acid added. v=0.2 V/s vs Fc⁺/Fc°.

FIG. 23. Overview of reactivity using non-innocent ligands.

FIG. 24. Electrochemical Characterization—CVs of CuL¹ in 0.1 M Bu₄NPF₆ ACN solution at scan rates of (from inside to outside, at peak) 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 V/s.CuL¹. (Inset: Cottrell plot of peak current vs square root of scan rate.)

FIG. 25. Homogeneous Catalytic Hydrogen Evolution: Cyclic Voltammetry (A) CVs (from bottom to top) of 0.6 mM CuL¹ in 0.1M Bu₄NPF₆ ACN with 0.0244 M CH₃COOH, 0.0448 M CH₃COOH, 0.0896 M CH₃COOH, 0.134 M CH₃COOH, 0.179 M CH₃COOH, 0.244 M CH₃COOH, and 0.269 M CH₃COOH, (Inset: Blow up of CV showing shift of Cu^(II/I) reduction event.); (B) Plot of i_(cat)/i_(p) vs [CH₃COOH] for 0.60 mM CuL¹ at scan rates of 0.20 (×), 0.50 (●) and 1.00 (*) V/s; (C) Catalytic Tafel Plot of CuL¹ with comparison of performance for hydrogen evolution with those of others reported in literature. ♦: Co^(II)(dmgH)₂py; ●: [Ni(P₂ ^(Ph)N^(Ph))₂]²⁺; ▪: NiL²; ▴:ZnL¹; ×:H₂L¹.

FIG. 26. Cyclic Voltammetry—(A) CVs (from bottom to top) of 0.6 mM CuL¹ in 0.1M Bu₄NPF₆ DMF with 0.0244 M CH₃COOH, 0.0672 M CH₃COOH, 0.112 M CH₃COOH, 0.157 M CH₃COOH, 0.202 M CH₃COOH, 0.246 M CH₃COOH, 0.269 M CH₃COOH, and 0.292 M CH₃COOH. (B) Plot of i_(cat)/i_(p) vs [CH₃COOH] for 0.6 mM CuL¹ at scan rates of 0.20 (♦), 0.50 (▪) and 1.00 (▴) V/s. (C) CVs of 0.6 mM CuL¹ in 0.1M Bu₄NPF₆ DMF with 0.292 M CH₃COOH at scan rates from 0.1 to 1.0 V/s. (D) Plot of cat vs scan rate for 0.6 mM CuL¹ in 0.1M Bu₄NPF₆ DMF with 0.292 M CH₃COOH.

FIG. 27. Kenetic Isotope Effects (KIE)—(A) CVs of 0.6 mM CuL¹ in 0.1 M Bu₄NPF₆ ACN solution with 0.269 M acetic acid at (from top to bottom) 0 mole % of CD₃COOD, 20 mole % of CD₃COOD, 40 mole % of CD₃COOD, 60 mole % of CD₃COOD, 80 mole % of CD₃COOD, and 100 mole % of CD₃COOD mole % of CD₃COOD. (B) Plot of KIE vs % CD₃COOD.

FIG. 28. Controlled Potential Electrolysis (CPE)—CPE of 0.6 mM CuL¹ in 0.1 M Bu₄NPF₆ ACN (left-most and second from left) or 0.1 M Bu₄NPF₆ DMF (second from right and right-most) solutions with 0.292 M CH₃COOH added; 0.1 M Bu₄NPF₆ DMF with 0.292 M CH₃COOH, no CuL¹ (overlaps with x-axis, zero apparent charge)).

FIG. 29. Control Experiments—(A) CVs run in 0.1 M Bu₄NPF₆ ACN solutions, showing blank ACN (bottom), with 0.0672 M acetic acid added (dashed—middle), and with 0.0672 M acetic acid and 0.6 mM CuL¹ (top). (B) CVs run in 0.1 M Bu₄NPF₆ DMF solutions, showing blank DMF (solid), with 0.0224 M acetic acid added (dashed), and with 0.0224 M acetic acid and 0.6 mM CuL¹ (top).

FIG. 30. Post-electrolysis “dip-test” of film—Performed on working electrode post CPE. Electrode washed with D.I water and immersed into a fresh solution of 0.1 M Bu₄NPF₆ ACN (bottom), and upon addition of 0.292 M CH₃COOH (top).

FIG. 31. Film analysis—Calculated (left bar) and XPS experimental (right bar) relative atomic mass percent for post-electrolysis CuL¹ derived films. Error bars show ±3σ for 4 experimental measurements.

FIG. 32. UV-Visible spectrum of CuL¹—titrated with CH₃COOH (from top to bottom); 0.022 M, 0.044 M, 0.056 M, 0.067 M, 0.089 M, 0.112 M, 0.134 M, 0.157 M, 0.202 M, 0.244 M, 0.269 M, 0.292 M, 0.337 M, and 0.382 M.

FIG. 33. ORTEP representation of [Cu(L¹H₂)(ClO₄)]ClO₄—Selected distances (Å): Cu—N1 1.9579(18), Cu—N4 1.9557(18), Cu—S1 2.2462(6), Cu—S2 2.2593(6), Cu—O1 2.5166(16), Cu . . . O7 2.9233(19), N2-H2n 0.78(3), N3-H3n 0.76(3), N4-N5 1.364(2), N5-H5n 0.78(2), N6-H6n 0.78(3). Selected angles (°): N1-Cu—N4 78.80(7), N1-Cu—S1 86.88(6), N4-Cu—S(2) 86.08(5), S1-Cu—S2 108.02(2), N2-N1-Cu 118.19(14), N1-N2-H2n 121(2), N5-N4-Cu 119.13(14), N4-N5-H5n 119.9(19).

FIG. 34. Proposed mechanism of hydrogen evolution by CuL¹.

FIG. 35. Energetic Stability from DFT—(A) Energetic stability of protonated species, [CuL¹H]⁺ (S=½). (B) Energetic Stability of protonated/reduced species, CuL¹H (S=0), B3LYP/6-311g(d,p).

FIG. 36. Energetic Stability from DFT—Energetic stability of the protonated/reduced/protonated species, [CuL¹H₂]⁺, in the singlet (S=0) and triplet (S=1) electronic states, B3LYP/6-311g(d,p).

FIG. 37. Energetic Stability from DFT—(A) Energetic stability of CuL¹H₂ (S=½). (B) Spin-density map of CuL.¹H₂ with second protonation on N4, B3LYP/6-311g(d,p).

FIG. 38. Qualitative frontier molecular orbital diagram highlighting site of reduction.

FIG. 39. Full ORTEP view of [Cu(L¹H₂)(ClO₄)]ClO₄.

FIG. 40. Open circuit potential measurement in 0.1 M Bu₄NPF₆ acetonitrile solution (top) and in 0.1 M Bu₄NPF₆ DMF solution (bottom) with 0.269 M acetic acid added.

FIG. 41. GC-TCD readout for gaseous product identification from electrolysis

FIG. 42. 0.6 mM CuL¹ in 0.1 M Bu₄NPF₆ acetonitrile HER CVs scanned at 0.2 V/s.

FIG. 43. 0.6 mM CuL¹ in 0.1 M Bu₄NPF₆acetonitrile HER CVs scanned at 0.5 V/s.

FIG. 44. 0.6 mM CuL¹ in 0.1 M Bu₄NPF₆ acetonitrile HER CVs scanned at 1.0 V/s.

FIG. 45. [CuL¹] dependence in 0.1 M Bu₄NPF₆ acetonitrile with 0.269 M CH₃COOH HER CVs.

FIG. 46. Plot of i_(p) vs [CuL¹] in 0.1 M Bu₄NPF₆ acetonitrile with 0.269 M CH₃COOH.

FIG. 47. 0.6 mM CuL¹ in 0.1 M Bu₄NPF₆ DMF; HER CVs scanned at 0.2 V/s.

FIG. 48. Plot of i_(cat)/i_(p) vs [CH₃COOH]; v=0.2 V/s.

FIG. 49. 0.6 mM CuL¹ in 0.1 M Bu₄NPF₆ DMF; HER CVs scanned at 0.5 V/s.

FIG. 50. Plot of i_(cat)/i_(p) vs [CH₃COOH]; v=0.5 V/s.

FIG. 51. 0.6 mM CuL¹ in 0.1 M Bu₄NPF₆ DMF; HER CVs scanned at 1.0 V/s.

FIG. 52. Plot of i_(cat)/i_(p) vs [CH₃COOH]; v=1.0 V/s.

FIG. 53. Plot of scan rate vs TOF for CuL¹ in DMF.

FIG. 54. Dip test post electrolysis in DMF.

FIG. 55. ¹H NMR spectrum of [CuL¹]⁻.

FIG. 56. ¹H NMR spectrum of [CuL¹H]⁺.

FIG. 57. Spin-density map of CuL¹ (S=1/2).

FIG. 58. Spin-density map of [CuL¹H]⁺.

FIG. 59. Energetic stability of the protonated/reduced species, CuL¹H, in the singlet (S=0) and triplet (S=1) electronic states.

FIG. 60. Full CV of CuL¹.

FIG. 61. High resolution (top) XPS of copper atoms and low resolution of entire adsorbed film (bottom).

FIG. 62. Post CV dip-test after 50 CV Cycles.

FIG. 63. Post CV dip-test after 50 CV Cycles.

FIG. 64. CV of Asymmetric Cu ligand—CVs of CuL^(10a) in CH₃CN with increasing [H⁺].

FIG. 65. GCE (Glass Carbon Electrode) polarization curves—Polarization curves in 0.5 H₂SO_(4 (aq)) with modified GCE (working), Pt (counter), and Ag/AgCl (3.5 M KCl) (reference) electrodes.

FIG. 66. Carbon Paste Electrode (CPE) analysis—(a) SEM image of CPE-CuL¹; (b) Nyquist plot of CPE-CuL¹; Polarization curves for modified CPEs.

FIG. 67. ORTEP view (50% probability) of compound 21—showing atom labeling for all non-hydrogen atoms in the asymmetric unit.

FIG. 68. ORTEP view (50% probability) depicting the cation of compound 22—showing atom labeling for all non-hydrogen atoms in the asymmetric unit. The counter anion iodide is omitted for clarity.

FIG. 69. ORTEP view (50% probability) of compound 22—showing atom labeling for all non-hydrogen atoms in the asymmetric unit.

FIG. 70. ORTEP view (50% probability) of compound 23—showing atom labeling for all non-hydrogen atoms in the asymmetric unit.

FIG. 71. ORTEP view (50% probability) of compound 23—showing atom labeling for all non-hydrogen atoms in the asymmetric unit. All hydrogen atoms are omitted for clarity.

FIG. 72. ORTEP view (50% probability) depicting the cation of compound 24—showing atom labeling for all non-hydrogen atoms in the asymmetric unit. The two counter anion iodides are omitted for clarity.

FIG. 73. ORTEP view (50% probability) of compound 24—showing atom labeling for all non-hydrogen atoms in the asymmetric unit.

FIG. 74. The unit cell (C2/c) of the cation of compound 22—iodide counter ion and hydrogens atoms are omitted for clarity.

FIG. 75. The unit cell (P-1) of the cation of compound 24—iodide counter ions and hydrogens atoms are omitted for clarity.

FIG. 76. Cyclic voltammetry for compounds 21 (blue) and 22 (red)—in the absence of substrate.

FIG. 77. Cyclic voltammetry for compounds 23 (blue) and 24 (red)—in the absence of substrate.

FIG. 78. CV of the first ligand centered reduction/oxidation event Ni(II)L/Ni(II)L⁻ in 21-24 (21—green; 22—orange; 23—blue; 24—red).

FIG. 79. The i_(cat)/i_(p) vs [CH₃COOH] plots for compounds 21 (blue-diamonds) and 22 (red-squares)—at the scan rate of 200 mV/s.

FIG. 80. The i_(cat)/i_(p) vs [CH₃COOH] plots for compounds 23 (blue-diamonds) and 24 (red-squares)—at the scan rate of 200 mV/s.

DETAILED DESCRIPTION

While embodiments encompassing the general inventive concepts may take diverse forms, various embodiments will be described herein, with the understanding that the present disclosure is to be considered merely exemplary, and the general inventive concepts are not intended to be limited to the disclosed embodiments.

In some embodiments, this application relates to inventive compounds (e.g., Formula (I), Formula (II), thiosemicarbazones and/or thiosemicarbazones and their metal (e.g., zinc, cobalt,or copper) complexes, and extended structures thereof), methods for preparation of the inventive compounds, compositions comprising the inventive compounds (e.g., anode, cathodes, catalysts (e.g., electrocatalysts), glassy carbon electrodes, carbon paste electrodes, covalently modified carbon (e.g., modified graphene)), electrochemical cells comprising compositions that comprise one or more inventive compounds, fuel cells comprising compositions that comprise one or more inventive compounds, uses of one or more inventive compounds to produce H₂ (e.g., via an electrochemical cell), and uses of one or more inventive compounds to create energy from H₂ (e.g., via a fuel cell). Additional embodiments of the invention are also discussed herein.

As used herein (unless otherwise specified), the term “alkyl” means a monovalent, straight or branched hydrocarbon chain. For example, the terms “C₁-C₇ alkyl” or “C₁-C₄ alkyl” refer to straight- or branched-chain saturated hydrocarbon groups having from 1 to 7 (e.g., 1, 2, 3, 4, 5, 6, or 7), or 1 to 4 (e.g., 1, 2, 3, or 4), carbon atoms, respectively. Examples of C₁-C₇ alkyl groups include, but are not limited to, methyl, ethyl, n-propyl, i-propyl, n-butyl, s-butyl, t-butyl, n-pentyl, s-pentyl, n-hexyl, and n-septyl. Examples of C₁-C₄ alkyl groups include, but are not limited to, methyl, ethyl, n-propyl, i-propyl, n-butyl, s-butyl, and t-butyl.

As used herein (unless otherwise specified), the term “alkenyl” means a monovalent, straight or branched hydrocarbon chain that includes one or more (e.g., 1, 2, 3, or 4) double bonds. Examples of alkenyl groups include, but are not limited to, vinyl, allyl, 1-propenyl, 2-propenyl, 1-butenyl, 2-butenyl, 3-butenyl, 1-pentenyl, 2-pentenyl, 3-pentenyl, 4-pentenyl, 1-hexenyl, 2-hexenyl, 3-hexenyl, 4-hexenyl, and 5-hexenyl.

As used herein (unless otherwise specified), the term “alkoxy” means any of the above alkyl groups which is attached to the remainder of the molecule by an oxygen atom (alkyl-O—). Examples of alkoxy groups include, but are not limited to, methoxy (sometimes shown as MeO—), ethoxy, isopropoxy, propoxy, and butyloxy.

As used herein (unless otherwise specified), the term “alkynyl” means a monovalent, straight or branched hydrocarbon chain that includes one or more (e.g., 1, 2, 3, or 4) triple bonds and that also may optionally include one or more (e.g. 1, 2, 3, or 4) double bonds in the chain. Examples of alkynyl groups include, but are not limited to, ethynyl, 1-propynyl, 2-propynyl, 1-butynyl, 2-butynyl, 3-butynyl, 1-pentynyl, 2-pentynyl, 3-pentynyl, 4-pentynyl, 1-hexynyl, 2-hexynyl, 3-hexynyl, 4-hexynyl, and 5-hexynyl.

As used herein (unless otherwise specified), the term “aryl” means a monovalent, monocyclic or bicyclic, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 member aromatic hydrocarbon group which, when unsubstituted. Examples of aryl groups include, but are not limited to, phenyl, naphthyl, pyrene, tolyl, and xylyl. For an aryl that is bicyclic, one or both rings can be substituted.

As used herein (unless otherwise specified), the term “cycloalkyl” means a monovalent, monocyclic or bicyclic, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12 membered hydrocarbon group. The rings can be saturated or partially unsaturated. Examples of cycloalkyl groups include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, and bicycloalkyls (e.g., bicyclooctanes such as [2.2.2]bicyclooctane or [3.3.0]bicyclooctane, bicyclononanes such as [4.3.0]bicyclononane, and bicyclodecanes such as [4.4.0]bicyclodecane (decalin), or spiro compounds). For a monocyclic cycloalkyl, the ring is not aromatic. For a bicyclic cycloalkyl, if one ring is aromatic, then the other is not aromatic. For a bicyclic cycloalkyl, one or both rings can be substituted.

As used herein (unless otherwise specified), the term “halogen” means monovalent Cl, F, Br, or I.

As used herein (unless otherwise specified), the term “heteroaryl” means a monovalent, monocyclic or bicyclic, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 membered, hydrocarbon group, where 1, 2, 3, 4, 5, 6, 7, 8, or 9 carbon atoms are replaced by a hetero atom independently selected from nitrogen, oxygen, or sulfur atom, and the monocyclic or bicyclic ring system is aromatic. Examples of heteroaryl groups include, but are not limited to, thienyl (or thiophenyl), furyl, indolyl, pyrrolyl, pyridinyl, pyrazinyl, oxazolyl, thiaxolyl, quinolinyl, pyrimidinyl, imidazolyl, 1-methyl-imidazolyl, triazolyl, tetrazolyl, 1H-pyrazol-4-yl, 1-Me-pyrazol-4-yl, pyridin-3-yl, pyridin-4-yl, 3,5-dimethylisoxazolyl, 1H-pyrrol-3-yl, 3,5-di-Me-pyrazolyl, and 1H-pyrazol-4-yl. For a bicyclic heteroaryl, if one ring is aryl, then the other is heteroaryl. For a bicyclic heteroaryl, one or both rings can have one or more hetero atoms. For a bicyclic heteroaryl, one or both rings can be substituted. An N-heteroaryl means a heteroaryl that comprises one or more N (e.g., 1, 2, 3, 4, 5, 6, 7, or 8); an N-heteroaryl may also comprise other hetero atoms.

As used herein (unless otherwise specified), the term “heterocyclyl” means a monovalent, monocyclic or bicyclic, 5, 6, 7, 8, 9, 10, 11, or 12 membered, hydrocarbon, where 1, 2, 3, 4, 5, or 6 carbon atoms are replaced by a hetero atom independently selected from nitrogen atom, oxygen atom, or sulfur atom, and the monocyclic or bicyclic ring system is not aromatic. Examples of heterocyclyl groups include, but are not limited to, tetrahydropyran, pyrolidinyl (e.g., pyrrolidin-1-yl, pyrrolidin-2-yl, pyrrolidin-3-yl, or pyrrolidin-4-yl), piperazinyl (e.g., piperazin-1-yl, piperazin-2-yl, piperazin-3-yl, or piperazin-4-yl), piperidinyl (e.g., piperadin-1-yl, piperadin-2-yl, piperadin-3-yl, or piperadin-4-yl), and morpholinyl (e.g., morpholin-1-yl, morpholin-2-yl, morpholin-3-yl, or morpholin-4-yl,). For a bicyclic heterocyclyl, if one ring is aromatic (e.g., monocyclic aryl or heteroaryl), then the other ring is not aromatic (e.g., a benzo crown ether). For a bicyclic heterocyclyl, one or both rings can have one or more hetero atoms. For a bicyclic heterocyclyl, one or both rings can be substituted. An N-heterocyclyl means a heterocyclyl that comprises one or more N (e.g., 1, 2, 3, 4, 5, or 6); an N-heterocyclyl may also comprise other hetero atoms.

As used herein (unless otherwise specified), the term “hetero atom” means an atom selected from nitrogen atom, oxygen atom, or sulfur atom.

As used herein (unless otherwise specified), the terms “hydroxy” or “hydroxyl” indicates the presence of a monovalent —OH group.

As used herein a “benzo crown ether” is benzene ring fused to a crown ether, such as but not limited to

“Benzo crown ether” is defined to encompass one, two, three, four, five, or six benzene rings fused to a crown ether. Any suitable crown ether can be used, such as but not limited to 12-crown-4, 15-crown-5, or 18-crown-6.

As used herein (unless otherwise specified), the term “substituted” (e.g., as in substituted alkyl) means that one or more hydrogen atoms of a chemical group (with one or more hydrogen atoms) can be replaced by one or more non-hydrogen substituents selected from the specified options. The replacement can occur at one or more positions. The term “optionally substituted” means that one or more hydrogen atoms of a chemical group (with one or more hydrogen atoms) can be, but is not required to be substituted.

Some compounds of the invention can have one or more chiral centers and can exist in and be isolated in optically active and racemic forms, for any of the one or more chiral centers. Some compounds can exhibit polymorphism. The compounds of the present invention encompass any optically active, racemate, stereoisomer form, polymorphism, or mixtures thereof. If a chiral center does not provide an indication of its configuration (i.e., R or S) in a chemical structure, it should be considered to represent R, S or a racemate.

Some embodiments of the invention include compounds of Formula (I),

and

salts, optical isomers, geometric isomers, salts of isomers, and derivatives thereof, wherein

—R¹ is

or is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—X¹ is bivalent —(NH)—, —O—, —(CH₂)—, or —S—, which —(NH)— or —(CH₂)— can optionally be substituted with one or more (e.g., 0, 1, or 2) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—R² is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl (e.g., benzo crown ether or pyrrolyl), heteroaryl (e.g., pyridinyl, imidazolyl, or 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl (e.g., benzo crown ether or pyrrolyl), heteroaryl (e.g., pyridinyl, imidazolyl, or 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), amine (—NH₂), —NR_(a)R_(b), —N⁽⁺⁾R_(a)R_(b)R_(c), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—R³ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—R⁴ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—R⁵ is

or is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—X² is bivalent —(NH)—, —O—, —(CH₂)—, or —S—, which —(NH)— or —(CH₂)— can optionally be substituted with one or more (e.g., 0, 1, or 2) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—R⁶ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl (e.g., benzo crown ether or pyrrolyl), heteroaryl (e.g., pyridinyl, imidazolyl, or 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl (e.g., benzo crown ether or pyrrolyl), heteroaryl (e.g., pyridinyl, imidazolyl, or 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), amine (—NH₂), —NR_(a)R_(b), —N⁽⁺⁾R_(a)R_(b)R_(c), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl; and

R_(a), R_(b), and R_(c) are each independently selected from C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl). If there are more than one of any of R_(a), R_(b), or R_(c) on the same compound (for example, if R² comprises an R_(a) and R⁶ also comprises an R_(a)), then each is chosen independently.

In some embodiments, one or both of R² or R⁶ is (a) C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl) substituted with —NR_(a)R_(b), (b) C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl) substituted with —N⁽⁺⁾R_(a)R_(b)R_(c), (c) substituted or unsubstituted benzo crown ether (e.g., mono benzo 18-crown-6 ether), (d) phenyl substituted with a carboxy (e.g., para, meta, or ortho substituted), (e) substituted or unsubstituted pyrrolyl (e.g., 2-amine or 5-amine substituted pyrrolyl), (f) substituted or unsubstituted pyridyl, or (g) substituted or unsubstituted imidazolyl.

In some embodiments, the compound of Formula (I) comprises one or more of the following:

(a) R³ is the same as R⁴;

(b) R³ is the same as R⁴ and R¹ is the same as R⁵;

(c) R³ is the same as R⁴, X¹ is —(NH)—, X² is —(NH)—, and R² is the same as R⁶;

(d) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NH)—, and R² is —CH₃;

(e) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NH)—, and R² is —C₅H₆;

(f) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NH)—, and R² is —CH₂F₃;

(g) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NCH₃)—, and R² is —CH₃;

(h) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —O—, and R² is —CH₃;

(i) R³ is the same as R⁴ and R³ is methyl;

(j) R³ is the same as R⁴ and R³ is ethyl;

(k) R³ is methyl and R⁴ is phenyl;

(l) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —(N—CH(CH₃)₂)—, and R⁶ is —CH(CH₃)₂;

(m) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —(NH)—, and R⁶ is —C₅H₆;

(n) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —(NH)—, and R⁶ is —CH₂CF₃;

(o) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —O—, and R⁶ is —CH₃;

(p) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —O—, and R⁶ is —CH₂CH₃;

(q) (1) the limitations of (l) and (2) the limitations of (i), (j), or (k);

(r) (1) the limitations of (m) and (2) the limitations of (i), (j), or (k);

(s) (1) the limitations of (n) and (2) the limitations of (i), (j), or (k);

(t) (1) the limitations of (o) and (2) the limitations of (i), (j), or (k); or

(u) (1) the limitations of (p) and (2) the limitations of (i), (j), or (k).

In some embodiments, Formula (I) further comprises a solvent molecule coordinated with Formula (I). In other embodiments, the solvent molecule can be any suitable solvent. In some embodiments, the solvent molecule is selected from water, methanol, ethanol, propanol, acetonitrile, dimethylformamide, and acetone.

In certain embodiments, the compound of Formula (I) is

where R⁵ is pyridinyl, 1-methyl-imidazolyl, an N-containing heterocyclyl, or an N-containing heteroaryl.

In some embodiments, Formula (I) is one of the molecules described in Example Sets A, B, C, D, or E. In other embodiments, Formula (I) can be symmetric or asymmetric.

In certain embodiments of Formula (I) (a) X¹ is not —(NH)—, (b) R² is not methoxyphenyl or p-methoxyphenyl, or (c) both (a) and (b). In other embodiments of Formula (I) (a) X² is not —(NH)—, (b) R⁶ is not methoxyphenyl or p-methoxyphenyl, or (c) both (a) and (b). In yet other embodiments, (a) R³ is not methyl, (b) R⁴ is not methyl, or (c) both (a) and (b). In still other embodiments, Formula (I) is not

In some embodiments, Formula (I) is part of a homogenous solution, a homogenous aqueous solution, a heterogeneous solution, a heterogeneous aqueous solution, or a glassy carbon electrode. In other embodiments, a homogenous solution, a homogenous aqueous solution, a heterogeneous solution, or a heterogeneous aqueous solution, can each comprise a compound of Formula (I). In certain embodiments, a glassy carbon electrode, a carbon paste (e.g., embedded with one or more of polynuclear catalysts, coordinated polymers, or metal-organic frameworks), covalent modified carbon (e.g., graphene), or non-covalent modified carbon (e.g., graphene), can each comprise or reacted with the compound of Formula (I).

Some embodiments of the invention include a compound selected from Formula (II),

M.L (II) and

salts, optical isomers, geometric isomers, salts of isomers, and derivatives thereof, wherein

-M is Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, Co, Rh, Ti, V, Cr, Mn, or Fe; or M is Cu²⁺, Cu⁺, Zn²⁺, Co²⁺, Ni²⁺, Cd²⁺, Mn²⁺, Ru²⁺, or Fe²⁺; or M is Cu²⁺, Cut, Zn²⁺, Co²⁺, Cd²⁺, Mn²⁺, Ru²⁺, or Fe²⁺; or M is Cu²⁺, Zn²⁺, Co²⁺, Cd²⁺, Mn²⁺, or Fe²⁺; or M is Cu²⁺, Zn²⁺, Ni²⁺, or Co²⁺; or M is Cu²⁺, Zn²⁺, or Co²⁺; or M is Cu²⁺, Zn²⁺, or Ni²⁺; or M is Cu²⁺ or Zn²⁺; and

-L is selected from a compound of Formula (I). In some embodiments, M includes one or more transition metals. In other embodiments, M does not include a transition metal. In certain embodiments, M includes non-transition metals.

In some embodiments, Formula (II) is a compound of Formula (II-A)

wherein

—R⁷ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl (e.g., benzo crown ether or pyrrolyl), heteroaryl (e.g., pyridinyl, imidazolyl, or 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl (e.g., benzo crown ether or pyrrolyl), heteroaryl (e.g., pyridinyl, imidazolyl, or 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), amine (—NH₂), —NR_(a)R_(b), —N⁽⁺⁾R_(a)R_(b)R_(c), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—X³ is bivalent —(NH)—, —(N—CH(CH₃)₂)—, —(N—CH₂CH₃)—, —(N—CH₃)—, or —O—, which —(NH)—, —(N—CH(CH₃)₂)—, —(N—CH₂CH₃)—, or —(N—CH₃)— can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—R⁸ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—R⁹ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—X⁴ is bivalent —(NH)—, —(N—CH(CH₃)₂)—, —(N—CH₂CH₃)—, —(N—CH₃)—, or —O—, which —(NH)—, —(N—CH(CH₃)₂)—, —(N—CH₂CH₃)—, or —(N—CH₃)— can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—R¹⁰ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl (e.g., benzo crown ether or pyrrolyl), heteroaryl (e.g., pyridinyl, imidazolyl, or 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl (e.g., benzo crown ether or pyrrolyl), heteroaryl (e.g., pyridinyl, imidazolyl, or 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), amine (—NH₂), —NR_(a)R_(b), —N⁽⁺⁾R_(a)R_(b)R_(c), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl; and

-M is Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, Co, Rh, Ti, V, Cr, Mn, or Fe; or M is Cu²⁺, Cu⁺, Zn²⁺, Co²⁺, Ni²⁺, Cd²⁺, Mn²⁺, Ru²⁺, or Fe²⁺; or M is Cu²⁺, Cut, Zn²⁺, Co²⁺, Cd²⁺, Mn²⁺, Ru²⁺, or Fe²⁺; or M is Cu²⁺, Zn²⁺, Co²⁺, Cd²⁺, Mn²⁺, or Fe²+; or M is Cu²⁺, Zn²⁺, Ni²⁺, or Co²+; or M is Cu²⁺, Zn²⁺, or Co²⁺; or M is Cu²⁺, Zn²⁺, or Ni²⁺; or M is Cu²⁺ or Zn²⁺.

In certain embodiments, M is Cu²⁺, Zn²⁺, Ni²⁺, or Co²⁺; or M is Cu²⁺, Zn²⁺, or Co²⁺; or M is Cu²⁺, Zn²⁺, or Ni²⁺; or M is Cu²⁺ or Zn²⁺. In some embodiments, M includes one or more transition metals. In other embodiments, M does not include a transition metal. In certain embodiments, M includes non-transition metals.

R_(a), R_(b), and R_(c) are each independently selected from C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl). If there are more than one of any of R_(a), R_(b), or R_(c) on the same compound (for example, if R⁷ comprises an R_(a) and R⁶ also comprises an R_(a)), then each is chosen independently.

In some embodiments, one or both of R⁷ or R¹⁰ is (a) C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl) substituted with —NR_(a)R_(b), (b) C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl) substituted with —N⁽⁺⁾R_(a)R_(b)R_(c), (c) substituted or unsubstituted benzo crown ether (e.g., mono benzo 18-crown-6 ether), (d) phenyl substituted with a carboxy (e.g., para, meta, or ortho substituted), (e) substituted or unsubstituted pyrrolyl (e.g., 2-amine or 5-amine substituted pyrrolyl), (f) substituted or unsubstituted pyridyl, or (g) substituted or unsubstituted imidazolyl.

In some embodiments of Formula (II),

(a) R³ is the same as R⁴;

(b) R³ is the same as R⁴ and R¹ is the same as R⁵;

(c) R³ is the same as R⁴, X¹ is —(NH)—, X² is —(NH)—, and R² is the same as R⁶;

(d) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NH)—, and R² is —CH₃;

(e) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NH)—, and R² is —C₅H₆;

(f) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NH)—, and R² is —CH₂F₃;

(g) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NCH₃)—, and R² is —CH₃;

(h) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —O—, and R² is —CH₃;

(i) R³ is the same as R⁴ and R³ is methyl;

(j) R³ is the same as R⁴ and R³ is ethyl;

(k) R³ is methyl and R⁴ is phenyl;

(l) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —(N—CH(CH₃)₂)—, and R⁶ is —CH(CH₃)₂;

(m) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —(NH)—, and R⁶ is —C₅H₆;

(n) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —(NH)—, and R⁶ is —CH₂CF₃;

(o) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —O—, and R⁶ is —CH₃;

(p) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —O—, and R⁶ is —CH₂CH₃;

(q) (1) the limitations of (l) and (2) the limitations of (i), (j), or (k);

(r) (1) the limitations of (m) and (2) the limitations of (i), (j), or (k);

(s) (1) the limitations of (n) and (2) the limitations of (i), (j), or (k);

(t) (1) the limitations of (o) and (2) the limitations of (i), (j), or (k); or

(u) (1) the limitations of (p) and (2) the limitations of (i), (j), or (k).

In some embodiments of Formula (II),

(a) R⁸ is the same as R⁹;

(b) R⁸ is the same as R⁹ and X³—R⁷ is the same as X⁴—R¹⁰;

(c) R⁸ is the same as R⁹, X³ is —(NH)—, X⁴ is —(NH)—, and R⁷ is the same as R¹⁰;

(d) R⁸ is the same as R⁹, R⁸ is methyl, X³—R⁷ is the same as X⁴—R¹⁰, X³ is —(NH)—, and R⁷ is —CH₃;

(e) R⁸ is the same as R⁹, R⁸ is methyl, X³—R⁷ is the same as X⁴—R¹⁰, X³ is —(NH)—, and R⁷ is —C₅H₆;

(f) R⁸ is the same as R⁹, R⁸ is methyl, X³—R⁷ is the same as X⁴—R¹⁰, X³ is —(NH)—, and R⁷ is —CH₂F₃;

(g) R⁸ is the same as R⁹, R⁸ is methyl, X³—R⁷ is the same as X⁴—R¹⁰, X³ is —(NCH₃)—, and R⁷ is —CH₃;

(h) R⁸ is the same as R⁹, R⁸ is methyl, X³—R⁷ is the same as X⁴—R¹⁰ X³ is —O—, and R⁷ is —CH₃;

(i) R⁸ is the same as R⁹ and R⁸ is methyl;

(j) R⁸ is the same as R⁹ and R⁸ is ethyl;

(k) R⁸ is methyl and R⁹ is phenyl;

(l) X³ is —(NH)—, R⁷ is —CH₃, X⁴ is —(N—CH(CH₃)₂)—, and R¹⁰ is —CH(CH₃)₂;

(m) X³ is —(NH)—, R⁷ is —CH₃, X⁴ is —(NH)—, and R¹⁰ is —C₅H₆;

(n) X³ is —(NH)—, R⁷ is —CH₃, X⁴ is —(NH)—, and R¹⁰ is —CH₂CF₃;

(o) X³ is —(NH)—, R⁷ is —CH₃, X⁴ is —O—, and R¹⁰ is —CH₃;

(p) X³ is —(NH)—, R⁷ is —CH₃, X⁴ is —O—, and R¹⁰ is —CH₂CH₃;

(q) (1) the limitations of (l) and (2) the limitations of (i), (j), or (k);

(r) (1) the limitations of (m) and (2) the limitations of (i), (j), or (k);

(s) (1) the limitations of (n) and (2) the limitations of (i), (j), or (k);

(t) (1) the limitations of (o) and (2) the limitations of (i), (j), or (k); or

(u) (1) the limitations of (p) and (2) the limitations of (i), (j), or (k).

In some embodiments of Formula (II), M is Zn²⁺, Co²⁺, Ni²⁺ or Cu²⁺, (e.g., M is Zn²⁺, Ni²⁺ or Cu²⁺; or M is Zn²⁺, Co²⁺ or Cu²⁺; or M is Zn²⁺ or Cu²⁺).

In other embodiments, Formula (II) further comprises a solvent molecule coordinated with Formula (II). In other embodiments, the solvent molecule can be any suitable solvent molecule. In certain embodiments, the solvent molecule is selected from water, ethanol, propanol, acetonitrile, dimethylformamide, and acetone.

In some embodiments, Formula (II) is

where M is Zn, Co, Ni, or Cu (e.g., M is Zn, Ni, or Cu; or M is Zn, Co, or Cu; or M is Zn or Cu) and R⁵ is pyridinyl, 1-methyl-imidazolyl, an N-containing heterocyclyl, or an N-containing heteroaryl.

In certain embodiments, Formula (II) is symmetric or is asymmetric.

In other embodiments, (a) X¹ is not —(NH)—, (b) R² is not methoxyphenyl or p-methoxyphenyl, or (c) both (a) and (b). In yet other embodiments, (a) X² is not —(NH)—, (b) R⁶ is not methoxyphenyl or p-methoxyphenyl, or (c) both (a) and (b). In certain embodiments, (a) R³ is not methyl, (b) R⁴ is not methyl, or (c) both (a) and (b).

In some embodiments, Formula (II) is not

In some embodiments, Formula (II) is

In other embodiments, Formula (II) is

In some embodiments, Formula (II) is one of the molecules described in Example Sets A, B, C, D, E, F, G, or H.

In some embodiments, a compound of Formula (II) is part of a homogenous solution, a homogenous aqueous solution, a heterogeneous solution, or a heterogeneous aqueous solution. In other embodiments, a homogenous solution, a homogenous aqueous solution, a heterogeneous solution, or a heterogeneous aqueous solution, each comprise a compound of Formula (II).

In other embodiments, a glassy carbon electrode, a carbon paste (e.g., embedded with one or more of polynuclear catalysts, coordinated polymers, or metal-organic frameworks), covalent modified carbon (e.g., graphene), or non-covalent modified carbon (e.g., graphene), each comprises or is reacted with a compound of Formula (II). In certain embodiments, the carbon paste carbon paste comprises an extended structure motif (e.g., motif I, motif II, or motif III of scheme below).

Certain embodiments of the invention include glassy carbon electrodes (GCE) comprising Formula (I), Formula (II), or both. Glassy carbon electrodes can be used for any suitable purpose including but not limited to use in heterogeneous aqueous solutions. Glassy carbon electrodes can be made using any suitable method including but not limited to dropcast or spray coating. In some instances, contributions to adhesion to the electrode can include but are not limited to π-π interactions, the water-insolubility of Formula (I) or Formula (II), or both. In some instances, asymmetric structures of Formula (I), Formula (II) or both can be used, in that the asymmetric structure is capable (e.g., designed) of linking to the GCE. In other instances, symmetric molecules of Formula (I), Formula (II) or both can be used to make GCEs.

Other embodiments of the invention include carbon paste electrodes (CPE) embedded with compositions comprising Formula (I), Formula (II), or both. The embedded compositions can be any suitable composition including but not limited to molecular catalysts or related extended structures (e.g., polynuclear catalysts, coordination polymers, metal-organic frameworks, or extended structures as described herein). Extended structures can be made using any suitable technique, including but not limited to those exemplified in the Examples or the scheme above; that technique can be applied to any suitable Formula (I) or Formula (II) and is not limited to the specific molecules used. In some instances, asymmetric structures of Formula (I), Formula (II) or both can be used, in that the asymmetric structure can be capable (e.g., designed) of forming desired extended networks. In other instances, symmetric molecules of Formula (I), Formula (II) or both can be used to make extended structures.

Still other embodiments of the invention include attachment of Formula (I), Formula (II), or both, to carbon surfaces (e.g., graphene, glassy carbon, graphite, carbon nanotubes, carbon nanospheres, or multiwalled carbon ananotubes). Attachment can include any suitable attachment including but not limited to covalent or enhanced non-covalent attachment (e.g., π-π interactions, insolubility of Formula (I) or Formula (II), or combinations thereof). In certain instances, the carbon surface (e.g., carbon electrode) can be modified with any suitable linker, such as being modified with diazonium compounds, amination chemistry, amide, coupling amines, carboxylic acids, epoxides or any suitable linker so that Formula (I), Formula (II), or an extended structure thereof can be linked to the carbon surface. Linker length, in some embodiments, is chosen to allow catalyst to adopt one or more confirmations that occur during catalysis (e.g., the linker can be used to force drive a desired catalytic mechanism, such as ligand centered reactivity). In yet other embodiments, the linker length is not so long as to diminish electron transfer between catalyst and carbon surface. In certain embodiments, Formula (I), Formula (II) or both can comprise a carboxylic acid or carboxaldehyde to attach to a carbon surface (e.g., modified with amines). In some instances, asymmetric structures of Formula (I), Formula (II) or both can be used, in that the asymmetric structure is capable (e.g., designed) of linking to the modified or unmodified carbon surface. In other instances, symmetric molecules of Formula (I), Formula (II) or both can be used (e.g., designed) to linking to the modified or unmodified carbon surface. In certain embodiments, related extended structures (e.g., polynuclear catalysts, coordination polymers, metal-organic frameworks, or extended structures as described herein) can be attached to the modified or unmodified carbon surface.

Covalent modification of carbon surfaces can be accomplished using any suitable technique, including but not limited to those exemplified in the scheme above; that technique can be applied to any suitable Formula (I) or Formula (II) and is not limited to the specific molecules used.

In some embodiments, the inventive compounds (e.g., Formula (I) or Formula (II), or their embodiments in any of the above GCEs, CPEs, or carbon surfaces) can be used in one or more of the following applications: catalysts (e.g., electrocatalysts) for activation of small molecules (e.g., alcohols, such as, but not limited to, methanol, ethanol, propanol, butanol, and all their isomers), integration into PEM fuel cells; hydrogen evolution for solar energy storage; sustainable hydrogen resource for fertilizer production; solid electrolytes for small battery development; hydrogenation/dehydrogenation catalysts; electrocatalytic CO₂ reduction catalyst; selective olefin binding and functionalization; ethylene and small molecule detection; desulfurization; incorporation into electroactivc films or thin films; incorporation into conductive polymers; electroactive, tunable metal-organic-frameworks; water purification; and water desalination.

In some embodiments, the inventive compounds (e.g., Formula (I) or Formula (II) and their metal (e.g., zinc or copper) complexes) can have one or more of the following uses or properties: use as a substitution for metal-hydride intermediates; capable of being engineered with different functional groups to match a desired application; air and water stable; no special precautions in preparation, storage, or handling; low molecular weights; capable of reducing the mass of catalyst in a particular application; capable of being prepared in high yield (e.g., in 4-7 steps from commercially available bulk reagents) and/or inexpensively; does not utilize precious or semi-precious metals for catalysts; catalysts can be prepared in alcohol solution; and catalysis can be conducted in alcohol or water.

In certain embodiments, the inventive (e.g., Formula (I) or Formula (II) and their metal (e.g., zinc or copper) complexes) comprise proton relay groups (e.g., as part of the ligand structure). In certain embodiments, the inventive compounds comprise structure(s) or modification(s) that are capable of attaching to an electrode surface, a solid support, or both. In certain instances, light driven evolution of H₂ with the inventive compounds can be accomplished using various photosensitizers and sacrificial redox mediators. In other embodiments, the inventive compounds (e.g., where M is Cu) can be used as a catalyst for the oxidation alcohols to aldehydes using air.

Some embodiments of the invention include a catalyst (e.g., an electrocatalyst) comprising a composition comprising a compound of Formula (I), a compound of Formula (II), or both. Other embodiments include an anode comprising a composition comprising a compound of Formula (I), a compound of Formula (II), or both. Other embodiments include a cathode comprising a composition comprising a compound of Formula (I), a compound of Formula (II), or both.

Other embodiments include an electrochemical cell comprising a composition comprising a compound of Formula (I), a compound of Formula (II), or both. In some instances, the cathode of the electrochemical cell comprises the composition, the anode of the electrochemical cell comprises the composition, or both.

Still other embodiments of the invention include a fuel cell comprising a composition comprising a compound of Formula (I), a compound of Formula (II), or both. In some instances, the cathode of the fuel cell comprises the composition, the anode of the fuel cell comprises the composition, or both.

Additional embodiments of the invention include a method for producing H₂ comprising contacting, in an electrochemical cell, a first composition comprising a compound of Formula (I), a compound of Formula (II), or both with a second composition comprising water. In some instances, the cathode of the electrochemical cell comprises the first composition. In certain embodiments, the Turn Over Frequency (TOF) is from about 20 s⁻¹ to about 100,000 s⁻¹, about 100 s⁻¹ to about 100,000 s⁻¹, from about 500 s⁻¹ to about 100,000 s⁻¹, from about 500 s⁻¹ to about 50,000 s⁻¹, from about 500 s⁻¹ to about 20,000 s⁻¹, about 20 s⁻¹, about 100 s⁻¹, about 500 s⁻¹, about 1000 s⁻¹, about 5000 s⁻¹, about 10000 s⁻¹, about 12000 s⁻¹, about 16000 s⁻¹, about 20000 s⁻¹, about 50000 s⁻¹, or about 100,000 s⁻¹. In some embodiments, the overpotential is greater than about 0 V, not less than about 0.1 V, not more than about 0.1 V, not more than 0.5 V, not more than 1 V, not more than 10 V, not more than 100 V, from about 0 V to about 2000 V, from about 0 V to about 1000 V, from about 0 V to about 750 V, from about 0 V to about 300 V, from about 0 V to about 350 V, from about 0 V to about 200 V, from about 0 V to about 100 V, from about 0 V to about 20 V, from about 0 V to about 10 V, from about 0 V to about 5 V, from about 0 V to about 2 V, from about 0 V to about 1 V, from about 0.1 V to about 2000 V, from about 0.1 V to about 1000 V, from about 0.1 V to about 750 V, from about 0.1 V to about 300 V, from about 0.1 V to about 350 V, from about 0.1 V to about 200 V, from about 0.1 V to about 100 V, from about 0.1 V to about 20 V, from about 0.1 V to about 10 V, from about 0.1 V to about 5 V, from about 0.1 V to about 2 V, from about 0.1 V to about 1 V, about 0.1 V, about 0.5 V, about 1 V, about 5 V, about 10 V, about 100 V, about 250 V, about 350 V, about 400 V, about 500 V, or about 1000 V.

Other instances of the invention include a method for oxidizing an aldehyde, an alcohol, acetonitrile, or water comprising contacting, in an electrochemical cell, a composition comprising a compound of Formula (I), a compound of Formula (II), or both. In some embodiments, the overpotential is greater than about 0 V, not less than about 0.1 V, not more than about 0.1 V, not more than 0.5 V, not more than 1 V, not more than 10 V, not more than 100 V, from about 0 V to about 2000 V, from about 0 V to about 1000 V, from about 0 V to about 750 V, from about 0 V to about 300 V, from about 0 V to about 350 V, from about 0 V to about 200 V, from about 0 V to about 100 V, from about 0 V to about 20 V, from about 0 V to about 10 V, from about 0 V to about 5 V, from about 0 V to about 2 V, from about 0 V to about 1 V, from about 0.1 V to about 2000 V, from about 0.1 V to about 1000 V, from about 0.1 V to about 750 V, from about 0.1 V to about 300 V, from about 0.1 V to about 350 V, from about 0.1 V to about 200 V, from about 0.1 V to about 100 V, from about 0.1 V to about 20 V, from about 0.1 V to about 10 V, from about 0.1 V to about 5 V, from about 0.1 V to about 2 V, from about 0.1 V to about 1 V, about 0.1 V, about 0.5 V, about 1 V, about 5 V, about 10 V, about 100 V, about 250 V, about 350 V, about 400 V, about 500 V, or about 1000 V.

Some embodiments of the invention include a method for producing electricity comprising contacting, in a fuel cell, a first composition comprising a compound of Formula (I), a compound of Formula (II), or both with a second composition comprising H₂. In some embodiments of this method, the anode of the fuel cell comprises the first composition. In some embodiments of this method, the cathode of the fuel cell comprises the first composition. In certain instances, the TOF is from about 1 s⁻¹ to about 1000 s⁻¹, from about 5 s⁻¹ to about 1000 s⁻¹, from about 5 s⁻¹ to about 500 s⁻¹, from about 5 s⁻¹ to about 200 s⁻¹, about 1 s⁻¹, about 5 s⁻¹, about 10 s⁻¹, about 32 s⁻¹, about 50 s⁻¹, about 76 s⁻¹, about 100 s⁻¹, about 120 s⁻¹, about 200 s⁻¹, about 300 s⁻¹, about 500 s⁻¹, or about 1000 s⁻¹. In yet additional embodiments, the overpotential is greater than about 0 V, not less than about 0.1 V, not more than about 0.1 V, not more than 0.5 V, not more than 1 V, not more than 10 V, not more than 100 V, from about 0 V to about 2000 V, from about 0 V to about 1000 V, from about 0 V to about 750 V, from about 0 V to about 300 V, from about 0 V to about 350 V, from about 0 V to about 200 V, from about 0 V to about 100 V, from about 0 V to about 20 V, from about 0 V to about 10 V, from about 0 V to about 5 V, from about 0 V to about 2 V, from about 0 V to about 1 V, from about 0.1 V to about 2000 V, from about 0.1 V to about 1000 V, from about 0.1 V to about 750 V, from about 0.1 V to about 300 V, from about 0.1 V to about 350 V, from about 0.1 V to about 200 V, from about 0.1 V to about 100 V, from about 0.1 V to about 20 V, from about 0.1 V to about 10 V, from about 0.1 V to about 5 V, from about 0.1 V to about 2 V, from about 0.1 V to about 1 V, about 0.1 V, about 0.5 V, about 1 V, about 5 V, about 10 V, about 100 V, about 250 V, about 350 V, about 400 V, about 500 V, or about 1000 V.

Other embodiments of the invention include a method for preparing a compound of Formula (I) comprising any suitable method, such as those disclosed herein. In some instances, the compound of Formula (I) is prepared comprising

(a) reacting a compound of Formula (III)

with a compound of Formula (IV)

(b) reacting a compound of Formula (V)

with a compound of Formula (VI)

and

(c) recovering the compound of Formula (I),

wherein R¹, R³, R⁴, and R⁵ are defined herein. Recovery can occur using any suitable method including but not limited to HPLC (e.g., reverse phase), LC, precipitation, centrifugation, column chromatography (e.g., size exclusion chromatography or ion exchange chromatography), use of silica gel, or combinations thereof.

Other embodiments of the invention include a method for preparing a compound of Formula (II) comprising any suitable method, such as those disclosed herein. In certain instances, the compound of Formula (II) is prepared comprising

(a) reacting a compound of Formula (I) with M (e.g., Zn, Co, or Cu) or salt thereof; and

(b) recovering the compound of Formula (II),

wherein M is defined herein. Recovery can occur using any suitable method including but not limited to HPLC (e.g., reverse phase), LC, precipitation, centrifugation, column chromatography (e.g., size exclusion chromatography or ion exchange chromatography), use of silica gel, or combinations thereof.

Additional embodiments include a method for preparing a catalyst (e.g., an electrocatalyst) comprising a composition comprising a compound of Formula (I), a compound of Formula (II), or both, comprising any suitable method, including those described herein. Additional embodiments include a method for preparing an anode comprising a composition comprising a compound of Formula (I), a compound of Formula (II), or both, comprising any suitable method, including those described herein. Further embodiments include a method for preparing a cathode comprising a composition comprising a compound of Formula (I), a compound of Formula (II), or both, comprising any suitable method, including those described herein.

The presently-disclosed subject matter is further illustrated by the following specific but non-limiting examples. The following examples may include compilations of data that are representative of data gathered at various times during the course of development and experimentation related to the present invention.

EXAMPLES Example Set A HER and HOR of Zn and Metal-Free Complexes

The compounds discussed in Example Set A include H₂L and ZnL.

Materials and Methods for Example Set A

All solvents were purified with an MBraun solvent purification system prior to use.

Electrochemical Methods

All cyclic voltammetry (CV) and controlled potential coulometry (CPC) measurements were recorded using a Gamry Interface potentiostat/galvanostat, which was connected to a glassy carbon working electrode (6.5 mm diameter, surface area=0.07 cm²), a platinum wire counter electrode, and Ag/AgCl reference electrode. Before use, the working electrode was polished using aqueous alumina slurry. Working and counter electrodes were cleaned before use by washing with water, ethanol, acetone, isopropanol and methanol, and then sonicated in methanol. CV measurements were conducted using a three-neck electrochemical cell that was washed and dried in oven over night before use. All electrochemical experiments were conducted under a N₂ atmosphere, aside from HOR experiments. All CPC measurements were conducted using a custom built gas tight Parr-electrolysis reactor with a volume of 30 mL washed and dried night before use. The working compartment was fitted with a platinum working electrode (surface area=0.07 cm²) and an Ag/AgCl reference electrode. The auxiliary compartment was fitted with a Pt wire counter electrode. The working compartment contained 12 mM acetic acid added to a 0.1 M Bu₄NPF₆ methanol solution, while the auxiliary compartment was filled with 0.1 M Bu₄NPF₆ methanol solution. Both compartments were purged for 15 min with N₂ and kept under a constant N₂ flow. A control (blank) CPC study was conducted and subtracted from experimental results. Electrolysis was then measured with the addition of the 0.1 mM ZnL. Electrolysis was conducted for 2.5 hours and the samples were subjected to gas chromatographic analysis every 30 minutes. A Gow-Mac series 400 GC-TCD with molecular sieve column was used for product detection. The column was heated to 130° C. under N₂ gas flow with 250 μL injection samples injected onto the column. The integrated area of the H₂ peak was then compared to the pre-made H₂ calibration curve in order to calculate the volume and moles of H₂ generated.

Statistical Analysis

Overpotential Determination: Overpotential can be defined as the difference between the thermodynamic and equilibrium potentials for a given reaction and the potential at which the reaction occurs under a set of specific conditions. Using this method of Appel and Helm (ACS Catal., 2014, Vol. 4, pp. 630-633; DOI: 10.1021/cs401013v), the overpotential (η) for proton reduction or H₂ oxidation by ZnL or H₂L under specific experimental conditions can be estimated as:

η=|(E _(OCP) −E _(cat/2))|

E_(OCP) is the measured open circuit potential measured under catalytic conditions specific for each reaction, and E_(cat/2) is the potential at one-half the maximum of the catalytic current measured for the catalyzed reduction of protons or oxidation of H₂ by ZnL or H₂L.

Overpotential calculation; ZnL HER:

η=Overpotential=|(E _(BH+(OCP)))−(E _(cat/2))|

η=|[−0.924−(−1.68V)]|

η=0.756 V vs Fc⁺/Fc⁰

Overpotential calculation; ZnL HOR:

η=Overpotential=|(E _(BH+(OCP)))−(E _(cat/2))|

η=|(0.190−0.505)|

η=0.315 V vs Fc⁺/Fc⁰

Overpotential calculation; H₂L HER:

η=Overpotential=|(E _(BH+(OCP)))−(E _(cat/2))|

η=|[−0.37−(−1.80)]|

η=1.43 V vs Fc⁺/Fc⁰

Overpotential calculation; H₂L HOR:

η=Overpotential=|(E _(BH+(OCP)))−(E _(cat/2))|

η=|(0.177−0.505)|

η=0.328 V vs Fc⁺/Fc⁰

Determination of ZnL Diffusion Coefficient (D₀):

Using the Randles-Sevcik equation (Eq. A2), and plotting peak current vs the square root of the scan rate allows for accurate calculation of the diffusion coefficient, D₀.

Slope (FIGS. 10-11)=1.94E−5=0.4463FA[cat][(FD₀/RT)]^(0.5)

-   -   A=0.071 cm²         -   [cat]=3E−6 moles/cm³     -   F=96485 C/mole e⁻         -   R=ideal gas constant         -   T=298 K         -   D₀=1.15E−7 cm²/s in MeOH

Sample Calculations Electrolysis:

Theoretical Moles of Hydrogen Made via Total Charge:

19.8 C×(1 mol e ⁻/96485 C)×(1 mol H₂/2 mol e ⁻)=moles H₂ theoretical

-   -   Moles H₂ theoretical=0.00011 moles H₂ based on charge from         electrolysis

Faradaic Efficiency Calculations:

Faradaic Efficiency=(Moles H₂ Quantified/Moles of H₂ Theoretical)×100%

Faradaic Efficiency=(0.000093 moles)/(0.00011 moles)×100%

-   -   Faradaic Efficiency=85%

TON Calculations:

TON=Moles of H₂ Produced/Moles of ZnL Used

TON=(0.00011 moles H₂ produced)/(0.000003 moles ZnL used)

-   -   TON=36.7

HER Equations for TOF Calculation:

Equation A1 details the relationship between the catalytic current i_(cat), the catalyst concentration [cat], and the acid concentration [H⁺] for a catalytic reaction that is first-order in acid and first-order in catalyst under scan rate independent conditions. The terms n, F, A, and D are the normal electrochemical terms related to the number of electrons transferred, Faraday's constant, area of the electrode, and diffusion constant, respectively.

i _(cat)nFA[cat]√{square root over (Dk[H⁺])}  (A1)

Equation A2 (Randle-Sevcik equation) provides the relationship between the peak current i_(p), catalyst concentration, and scan rate (v) in the absence of acid. The factor of 0.4463 is related to the diffusion equations, R is the gas constant, and T is temperature in K. The other terms are the same as in equation A1.

$\begin{matrix} {i_{p} = {0.4463\; {{FA}\lbrack{cat}\rbrack}\sqrt{\frac{FvD}{RT}}}} & ({A2}) \end{matrix}$

Thus, the ratio of i_(cat)/i_(p) (equation A3) is obtained from equations A1 and A2

$\begin{matrix} {\frac{i_{cat}}{i_{p}} = {\frac{n}{0.4463}\sqrt{\frac{R\; T\; {k\left\lbrack H^{+} \right\rbrack}}{F\; v}}}} & ({A3}) \end{matrix}$

Under pseudo first-order conditions where k_(obs)=k[H⁺], equation A3 simplifies to A4.

$\begin{matrix} {\frac{i_{cat}}{i_{p}} = {\frac{n}{0.4463}\sqrt{\frac{R\; T\; k_{obs}}{F\; v}}}} & ({A4}) \end{matrix}$

Equation A4 can further be simplified to equation A5, when n=1 for bimolecular processes.

$\begin{matrix} {k_{obs} = {v \times \left\lbrack \frac{\frac{i_{cat}}{i_{p}}}{0.35} \right\rbrack^{2}}} & ({A5}) \end{matrix}$

Since no peak current for ZnL was observed in the absence of substrate in methanol, the experimentally determined diffusion coefficient, 1.15E-7, was used to calculate the value for i_(p). This gave an i_(p) of 43 μA when run at 5 V/s (the scan rate in which catalytic current becomes independent of scan rate). Furthermore, the value of i_(p) was confirmed through simulations using DigiElch, which agree with the calculated i_(p) values. Using equation A5, the TOF or k_(obs) can be calculated using the experimentally determined i_(p) value as well as the icat observed at 5 V/s, 230 μA. This results in a TOF of 1170 s⁻¹.

We then calculated the TOF using Eq. A6, which is Eq. A1 under pseudo first-order conditions, in order to compare both calculated values, which are in agreement with each other.

i _(cat)=nFA[cat]√{square root over (Dk_(obs))}  (A6)

Sample Calculations ZnL TOF/kobs:

Using Eq. A5:

i _(cat)=230 μA; i _(p)=43 μA; bv=5.0 V/s

=k _(obs)/TOF=1170 s ⁻¹

Using Eq. A6:

i _(cat)=230 μA; n=1 mole e ⁻/mole of ZnL; F=96485 C/mole⁻ ; A=0.071 cm²; [cat]=3E−6 moles/cm³ ; D _(cat)=1.15E−7 cm² /s.

=k _(obs)/TOF=1100 s ⁻¹

ZnL HER Kinetic Isotope Effect: Acetic Acid vs d-Acetic Acid:

Scan Rate k_(H) k_(D) k_(H)/k_(D) 5 V/s 1170 975 1.2

HOR TOF ZnL and H₂L Sample Calculation when v=1.0 V/s:

ZnL:  TOF = k_(obs) = v * 1.94(i_(cat)/i_(p))²  when  i_(cat) = −712  μ A  and  i_(p) = −117  μ A  at  1.0  V/s   TOF = 72  s⁻¹ H₂L:  TOF = k_(obs) = v * 1.94(i_(cat)/i_(p))²  when  i_(cat) = −475  μ A  and  i_(p) = −117  μ A  at  1.0  V/s   TOF = 32  s⁻¹

Computational Methods

All calculations were performed in the gas phase using density functional theory (DFT) employing the B97-D exchange correlation functional, and the 6-311G(d) basis set for all atoms as implemented in the Gaussian09 suite of programs for electronic structure and ChemCraft was used for graphics visualization. Transition states were determined locally using the Berny algorithm with GEDIIS, and verified by IRC calculations with forward and reverse step sizes of 40. All optimizations were performed under tight constraints, with no symmetry imposed. Several dimeric TS structures in various protonation states were initially investigated by DFT using the berny algorithm for local TS optimization in the gas phase. These structures were constructed manually based on optimized reactants and products, or by modifying previously published semicarbazide dimers. Dimers without ruptured Zn—S and Zn—N bonds were also considered, but precluded based on energetic grounds.

Supplementary Text

Blank and control experiments were performed for ZnL and H₂L HER CV studies. Blank runs consisted of 0.1 M Bu₄NPF₆ methanol or acetonitrile, depending on experiment, which had been purged with N₂ gas for 10 minutes. Control CVs run in 0.1 M Bu₄NPF₆ methanol or acetonitrile with 12 mM acetic acid showed minimal currents when compared to currents observed after addition of either ZnL or H₂L electrocatalysts.

Blank and control experiments were performed for ZnL and H₂L HOR CV studies. Blank runs consisted of 0.1 M Bu₄NPF₆ methanol solutions, which had been purged with N₂ gas for 10 minutes. Control CVs in the absence of ZnL or H₂L were performed. CVs were run under an H₂ atmosphere in solutions of 0.1 M Bu₄NPF₆ methanol with increasing concentrations of triethylamine, added until a concentration of 30 mM. The current observed was significantly lower when compared to the current observed after the addition of the ZnL or H₂L electrocatalysts. Additionally, control experiments were performed with ZnL or H₂L in 0.1 M Bu₄NPF₆ methanol solutions under an N₂ atmosphere. Application of an N₂ atmosphere resulted in no catalytic currents. After introduction of an H₂ atmosphere and purging the solution with H₂ for 15 minutes, catalytic current was observed.

To quantify H₂ production, the output gas was sampled, 250 μL, every 30 minutes and analyzed by the GC-TCD described in electrochemical methods section. After sampling, the chromatographic peak area of hydrogen is obtained. The GC-TCD calibration curve was prepared by sampling known hydrogen concentrations, made with known volumes of hydrogen, from the working compartment, with a constant known N₂ flow rate, and then measured by the same procedure described above. A linear relationship between the chromatographic peak areas of the hydrogen sampled and the specific amounts of hydrogen used was established, defined by y=mx+b, where y is the peak area and x is the amount of hydrogen. Using this linear relationship, the amount of hydrogen produced during experimental electrolysis can be calculated from the integrated peak areas obtained.

Digital simulations of voltammetric data were performed using commercially available DigiElch Pro software package (v.7). Models were fit using an experimentally determined ZnL diffusion coefficient and an experimentally determined value of α and k_(s). The consistency of the mechanism over a broad set reaction conditions was confirmed through models employing multiple scan rates and acid concentrations, all which agree with experimental results

Examination of the change in bond lengths and bond angles amongst ZnL, [Zn(HL)]⁺, Zn(HL.), and [Zn(H₂L.)]⁺ (Tables A4-A6) assist to explain structural and electronic changes over the course of the ZnL catalyzed HER mechanism. Initial protonation of ZnL to give [Zn(HL)⁺] results in a slight puckering of the ligand framework around the Zn center shown by the lengthening of the Zn—S1, Zn—N2, Zn—N3 bonds and a decrease in the Zn—S2 bond as well as an increase in the S1-Zn—S2, N3-Zn—S2 bond angles and decrease of the S1-Zn—N2, N2-Zn—N3 bond angles. Subsequent reduction to the neutral radical species, Zn(HL.), is accompanied by significant contraction of the Zn—N2 and Zn—N3 bonds, 2.118 Å and 2.125 Å to 2.059 Å and 2.045 Å, respectively. Furthermore, moving across the mechanism from protonation to reduction, the C2-C3 bond length always decreases in length moving from an initial length of 1.478 Å to 1.470 Å after protonation, and then decreasing further to 1.427 Å after reduction, in agreement with the spin-density map of Zn(HL.).

Results and Discussion for Example Set A

Solutions of ZnL in methanol or acetonitrile display catalytic hydrogen evolution upon reduction in the presence of acetic acid. In methanol, the cathodic current at −1.7 V increases with increasing acid concentration indicative of an electrocatalytic process (FIG. 1A). The current plateaus at 12.0 mM acetic acid indicating acid-saturation (FIG. 1B) with a maximum turnover frequency (TOF) of 1170 s⁻¹ at overpotential of 756 mV. No reduction wave for ZnL is observed within the potential limits of methanol in the absence of acid, signifying that HER might require protonation prior to reduction. In acetonitrile, addition of acetic acid results in catalytic current at −2.3 V, which is near the irreversible ligand-centered reduction of H₂L in the absence of acid (FIGS. 4-5) and within the range of reduction potentials previously reported for thiosemicarbizides. Catalytic current becomes independent of acid concentration at 23 mM, yielding a higher TOF of 11700 s⁻¹, but with a larger overpotential of 1074 mV. The lower overpotential in methanol appears consistent with outer-coordination sphere proton shuttling, which facilitates ligand protonation prior to electrochemical reduction. The HER TOF of ZnL is substantially higher than other proposed ligand-centered catalysts suggesting H₂L itself may also demonstrate catalytic activity.

The metal-free H₂L ligand was subsequently evaluated as a proton reduction catalyst. H₂L displays an irreversible reduction at −2.1 V and an irreversible oxidation at +0.5 V in methanol versus Fc⁺/Fc. Upon addition of acetic acid, the cathodic current at −2.1 V increases steadily (FIG. 6) reaching a maximum at concentrations of 9.8 mM (FIG. 1B). Under acid-saturated conditions, H₂L displays a TOF of 1320 s⁻¹ with an overpotential of 1430 mV. To our knowledge, this is the only reported metal-free, homogeneous electrocatalyst for HER.

As well as electrocatalytic HER, ZnL and H₂L also catalyze HOR. Introduction of triethylamine to methanol solutions of ZnL or H₂L under one atmosphere of H₂ results in an increase in anodic current near the irreversible oxidation wave of ZnL or H₂L, respectively (FIG. 1C and FIG. 7). For ZnL, the catalytic current shows saturation behavior (FIG. 1D) with near saturation at a base concentration of 30 mM yielding a TOF of 72 s⁻¹ with an overpotential of 315 mV. The HOR activity of H₂L ligand was similarly assessed reaching saturation at 21 mM base (FIG. 1D) with a TOF of 32 s⁻¹ and an overpotential of 328 mV. The HOR TOFs of ZnL and H₂L are among the highest reported of any homogenous electrocatalyst.

The stability of ZnL as a HER electrocatalyst was further examined by controlled potential coulometry. At an applied potential of −1.7 V versus Fc⁺/Fc, ZnL evolves H₂ from 12 mM acetic acid solutions in methanol with a turnover number (TON) of 37 after 2.5 hours (FIG. 2A) based on a total charge of 19.8 C. The identity of the gaseous product was confirmed as H₂ by gas chromatography thermal conductivity (GC-TCD). The integrated peak areas of headspace samples collected during electrolysis (FIG. 8) indicate a minimum faradaic efficiency of 85%. Throughout the electrolysis, the TOF remained consistent at 15 h⁻¹ with no signs of decreasing activity. Spectroelectrochemical experiments were performed on 0.1 M Bu₄NPF₆ methanol solutions of ZnL with an applied potential of −1.7 V in order to identify the absorption characteristics of the one-electron reduced electrocatalyst, [ZnL]⁻. UV-Vis spectra were recorded before electrolysis and then measured every 15 minutes during electrolysis showing the growth of the absorption band near 250 nm and a decrease in the absorption band near 430 nm (FIG. 21). A CV was then recorded with addition of 12 mM acetic acid (FIG. 22). An additional control was performed after prolonged reduction in order to rule out ligand decomposition onto electrode surface as possible source of catalysis. After reduction, the working electrode was removed, washed with DI water, and then placed in fresh solution containing no catalyst, upon which no current was observed.

To evaluate the HER mechanism of ZnL, we first determined the rate law and measured the H/D kinetic isotope effect. Under acid-dependent conditions, the catalytic current (i_(cat)) displays a linear dependence on the square root of the scan rate indicating the current appears limited by acid diffusion to the electrode surface (FIGS. 9-10). Further, under non-saturating acid conditions cat appears directly proportional to [H⁺] (FIG. 1B) indicating a first-order dependence on acid concentration. Varying the [ZnL] at fixed acid concentrations confirms first-order dependence at catalyst concentrations above 2 mM (FIGS. 11-12). Using the deuterated acid CD₃CO₂D, the ZnL catalyst displays a small kinetic isotope effect (KIE) of 1.2.

Several example simulations and calculations were performed, as described herein. However, the scope of the invention is not limited by the results, pathways, or mechanisms exemplified in the simulations.

Digital simulations of the cyclic voltammograms (FIG. 2B and Table A1) reveal parallel routes to proton reduction involving homo-coupling of two, neutral Zn(HL^(⋅)) radicals and hetero-coupling of a neutral Zn(HL^(⋅)) radical with the cationic radical [Zn(H₂L^(⋅))]⁺. The proposed mechanism (FIG. 2C) begins with protonation of ZnL, K=2.4×10⁵, followed by reduction to Zn(HL^(⋅)), E°=−1.81 V vs. Fc⁺/Fc. In the homo-coupling pathway, two Zn(HL^(⋅)) rapidly combine, k_(f)=3×10⁹M⁻¹ s⁻¹, to evolve H₂ and regenerate two equivalents of ZnL. In the alternate pathway, one equivalent of Zn(HL^(⋅)) is further protonated, K=8.8 , prior to hetero-coupling. Combination of [Zn(H₂L^(⋅))]⁺ with the second equivalent of Zn(HL^(⋅)), k_(f)=2×10¹⁰ M⁻¹ s⁻¹, yields H₂ completing the catalytic cycle. The simulated kinetic and thermodynamic parameters reveal that both routes to H₂ evolution are operational across a range of experimental conditions (FIGS. 13-15 and Table A2-A3).

Density functional theory (DFT) calculations using the B97-D functional and the 6-311G(d) basis set, support the proposed catalytic cycle and elucidate the hydrazino nitrogen as the site of protonation. Each of the metal complexes in FIG. 2C was successfully optimized (Table A4-A6). Energies (Table A7) reveal that protonation at the hydrazino nitrogen (FIG. 16) is favored by at least 13.0 kcal/mol relative to other potential basic sites within ZnL (FIGS. 17-19). Evolution of H₂ through homo-coupling of two Zn(HL^(⋅)) radicals is exergonic by 42.6 kcal/mol, while the parallel pathway involving hetero-coupling of Zn(HL^(⋅)) and [Zn(H₂L^(⋅))]⁺ releases 28.8 kcal/mol.

Analyses of the Zn(HL^(⋅)) and [Zn(H₂L^(⋅))]⁺ spin density profiles (FIGS. 3A and 3B) show radical character delocalized on both protonated ligand frameworks. H₂ is evolved by radical hetero-coupling, overcoming an 8.1 kcal/mol barrier (FIGS. 3C and 3D). The absence of spin density on Zn for all species involved in the HER, is support for ligand based reduction (Table A8). The transition state (TS) can be described as a dimer with H dissociations from each monomer fragment, along their respective N—H coordinates to form H₂ (FIG. 20). This is consistent with N-H bond lengths in the TS of 1.25 Å for Zn(HL^(⋅)), and 1.36 Å for [Zn(H₂L^(⋅))]⁺ compared to respective equilibrium N-H distances, both of 1.02 Å. The longer N—H bond in the TS associated with [Zn(H₂L^(⋅))]⁺ may also be attributed to an increased charge density along the forward IRC for both N and H, compared to Zn(HL^(⋅)) (FIG. 20). The HER from [Zn₂H₃L^(⋅) ₂]⁺ is thus interpreted as dimeric, where the now charge-reorganized Zn(H₂L^(⋅)) fragment promotes early electron transfer, and is coupled to proton transfer from [Zn(HL^(⋅))]⁺ to form H₂.

In summary, some of the non-transition metal complex ZnL and the metal-free ligand H₂L disclosed herein appear to represent a fundamentally new class of homogeneous HER and HOR electrocatalysts. Unlike traditional catalysts that employ a metal-hydride as the key intermediate, this new approach facilitates H₂ evolution through ligand-centered radical coupling. The combination of the redox active ligand H₂L with the non-transition metal Zn constrains redox activity to the ligand, in contrast to transition metal complexes where spin-coupling between the ligand radical and unpaired electrons on the metal may reduce reactivity. The confinement of radical character to the ligand is further evidenced by the catalytic activity of H₂L; albeit with higher overpotential than ZnL. The enhanced activity with Zn, in some instances, is understood to be attributed in part to the Lewis acidity of Zn(II), which balances the charge of the anionic ligand, promotes protonation, and lowers the reduction potential. Further, Zn(II) can provide a structural framework for the N₂S₂ chelate that pre-organizes the radical complexes for H₂ evolution.

TABLE A1 Optimized parameters of data fitting, 12 mM [acid]; ν = 0.2-0.5 V/s vs Fc⁺/Fc⁰ 99% Charge-transfer Steps Confidence E⁰ α k_(s) [ZnHL]⁺+ e⁻ = Zn(HL·) Optimized −1.8110 0.3166 0.0070 Upper Limit −1.8113 0.3166 0.0070 Lower Limit −1.8107 0.3166 0.0070 [ZnH₂L]²⁺ + e⁻ = [Zn(H₂L·)]⁺ Optimized −1.5872 0.3166 0.0070 Upper Limit −1.5874 0.3166 0.0070 Lower Limit −1.5870 0.3166 0.0070 99% Chemical Steps Confidence K_(eq) k_(f) ZnL + H⁺ = [ZnHL]⁺ Optimized 2.42E+05 1.28E+13 Upper Limit 3.42E+06 6.40E+13 Lower Limit 1.91E+05 1.94E+12 [ZnHL]⁺ + H⁺ = [ZnH₂L]²⁺ Optimized 8.80E+00 4.06E+06 Upper Limit 8.93E+00 6.92E+06 Lower Limit 8.68E+00 1.20E+06 Zn(HL·) + Zn(HL·) = H₂ Optimized 4.89E+10 3.09E+09 Upper Limit 4.96E+10 6.51E+09 Lower Limit 4.80E+10 6.45E+08 Zn(HL·) + [Zn(H₂L·)]⁺ = H₂ Optimized 9.07E+07 2.47E+10 Upper Limit 9.19E+07 3.95E+10 Lower Limit 8.90E+07 9.94E+09 [Zn(H₂L·)]⁺ = Zn(HL·) + H⁺ Calculated 1.87E−05 8.14E+04 Upper Limit 1.87E−05 1.07E+05 Lower Limit 1.87E−05 2.82E+04

TABLE A2 Optimized parameters of data fitting, 12 mM [acid]; ν = 0.6-5.0 V/s vs Fc⁺/Fc⁰ 99% Charge-transfer Steps Confidence E⁰ α k_(s) [ZnHL]⁺ + e⁻ = Zn(HL·) Optimized −1.8004 0.3166 0.007 Upper Limit −1.8010 0.3166 0.007 Lower Limit −1.8000 0.3166 0.007 [ZnH₂L]²⁺ + e⁻ = [Zn(H₂L·)]⁺ Optimized −1.5264 0.3166 0.007 Upper Limit −1.5274 0.3166 0.007 Lower Limit −1.5254 0.3166 0.007 99% Chemical Steps Confidence K_(eq) k_(f) ZnL + H⁺ = [ZnHL]⁺ Optimized 1364 3.20E+13 Upper Limit 1442.8 8.72E+15 Lower Limit 1285.2 8.65E+12 [ZnHL]⁺ + H⁺ = [ZnH₂L]²⁺ Optimized 13.438 3.06E+09 Upper Limit 13.6 9.83E+10 Lower Limit 13.276 9.22E+08 Zn(HL·) + Zn(HL·) = H₂ Optimized 3.69E+11 4.57E+08 Upper Limit 3.69E+11 5.57E+08 Lower Limit 3.68E+11 3.57E+08 Zn(HL·) + [Zn(H₂L·)]⁺ = H₂ Optimized 4.44E+08 2.72E+11 Upper Limit 4.46E+08 3.84E+11 Lower Limit 4.42E+08 1.61E+11 [Zn(H₂L·)]⁺ = Zn(HL·) + H⁺ Calculated 1.74E−06 2414 Upper Limit 1.74E−06 2414 Lower Limit 1.74E−06 2414

TABLE A3 Optimized parameters of data fitting, 6 mM [acid]; ν = 0.2-0.5 V/s vs Fc⁺/Fc^(o) 99% Charge-transfer Steps Confidence E⁰ α k_(s) [ZnHL]⁺ + e⁻ = Zn(HL·) Optimized −1.8431 0.3166 0.007 Upper Limit −1.8562 0.3166 0.007 Lower −1.8333 0.3166 0.007 Limit [ZnH₂L]²⁺ + e⁻ = [Zn(H₂L·)]⁺ Optimized −1.6958 0.3166 0.007 Upper Limit −1.7939 0.3166 0.007 Lower −1.5977 0.3166 0.007 Limit 99% Chemical Steps Confidence K_(eq) k_(f) ZnL + H⁺ = [ZnHL]⁺ Optimized 32000 5.00E+11 Upper Limit 6.68E+05 1.43E+12 Lower 6.04E+03 1.04E+10 Limit [ZnHL]⁺ + H⁺ = [ZnH₂L]²⁺ Optimized 19.942 2.86E+02 Upper Limit 95.717 2.86E+03 Lower 0.55833 2.23E+01 Limit Zn(HL·) + Zn(HL·) = H₂ Optimized 7.28E+09 2.00E+08 Upper Limit 1.05E+11 1.35E+12 Lower 9.08E+08 1.35E+06 Limit Zn(HL·) + [Zn(H₂L·)]⁺ = H₂ Optimized 8.58E+07 2.00E+12 Upper Limit 1.28E+09 1.49E+13 Lower 1.11E+06 1.09E+10 Limit [Zn(H₂L·)]⁺ = Zn(HL·) + H⁺ Calculated 1.55E−04 4000 Upper Limit 1.55E−04 4754.6 Lower 1.55E−04 3245.4 Limit

TABLE A4 Bond length comparison of calculated HER intermediates Structures ZnL [ZnHL]⁺ Zn(HL•) [ZnH₂L•]⁺ Bond Lengths ( Å ) Zn—S1 2.368 2.423 2.458 2.386 Zn—S2 2.368 2.318 2.341 2.386 Zn—N2 2.116 2.118 2.059 2.061 Zn—N3 2.116 2.125 2.045 2.061 S1—C1 1.774 1.719 1.728 1.732 C1—N5 1.356 1.343 1.373 1.349 N5—C5 1.460 1.467 1.459 1.463 C1—N1 1.339 1.373 1.351 1.361 N1—N2 1.344 1.357 1.368 1.362 N1—H15 — 1.015 1.016 1.015 N2—C2 1.312 1.313 1.368 1.348 C2—C3 1.478 1.470 1.427 1.436 C3—N3 1.312 1.322 1.345 1.348 N3—N4 1.344 1.321 1.349 1.362 N4—C4 1.339 1.361 1.329 1.361 C4—S2 1.774 1.767 1.792 1.732 C4—N6 1.356 1.341 1.367 1.349 N6—C8 1.460 1.469 1.458 1.463

TABLE A5 Bond angle comparison of calculated HER intermediates Structures ZnL [ZnHL] Zn(HL•) [ZnH₂L•]⁺ Bond Angles (°) S1—Zn—S2 118.02 119.08 116.68 117.56 S1—Zn—N2 82.78 81.57 82.39 83.80 N2—Zn—N3 76.43 74.94 77.40 73.34 N3—Zn—S2 82.78 84.41 85.07 83.30 C1—S1—Zn 93.70 96.92 95.10 95.54 N1—N2—C2 121.88 122.99 120.43 122.41 N1—N2—Zn 122.08 119.62 119.13 118.45 C2—N2—Zn 116.04 117.39 115.19 118.59 C3—N3—Zn 116.04 117.76 117.15 118.59 C3—N3—N4 121.88 122.01 121.05 122.41 N4—N3—Zn 122.08 120.23 121.44 118.45

TABLE A6 Computational Input Coordinates ZnL 0 1 Zn  0.00003000 −0.87035000 −0.00021000 S  1.99717800 −1.98187000  0.39859600 C  3.02385700 −0.57568000  0.06312600 N  2.64126500  0.68182000 −0.11295000 N  1.29611500  0.83221600 −0.13343000 C  0.74221000  2.00306700 −0.01927000 C −0.74227000  2.00309000  0.01908300 N −1.29622000  0.83222900  0.13305300 N −2.64133000  0.68180300  0.11294100 C −3.02384000 −0.57582000 −0.06295000 S −1.99721000 −1.98199000 −0.39809000 N  4.35570700 −0.82176000  0.01195100 C  5.36070300  0.19708100 −0.24871000 C  1.51592800  3.28691200  0.08674400 C −1.51582000  3.28710900 −0.08627000 N −4.35572000 −0.82183000 −0.01172000 C −5.36071000  0.19724100  0.24805300 H  4.64910400 −1.76944000  0.19775300 H −4.64921000 −1.76953000 −0.19732000 H  6.34217200 −0.28345000 −0.24323000 H  5.33720300  0.97919000  0.51882200 H  5.19661500  0.66931400 −1.22332000 H  2.57356100  3.10001400 −0.10327000 H  1.41553600  3.72317600  1.09013800 H  1.14601100  4.03184200 −0.62751000 H −2.57412000  3.09947900  0.09927500 H −1.14881000  4.02987600  0.63178900 H −1.41150000  3.72652300 −1.08785000 H −6.34227000 −0.28309000  0.24188600 H −5.19732000  0.66960500  1.22273300 H −5.33643000  0.97923600 −0.51956000 [ZnHL]⁺ 1 1 Zn  0.000030000 −0.870350000 −0.000210000 S  1.997178000 −1.981870000  0.398596000 C  3.023857000 −0.575680000  0.063126000 N  2.641265000  0.681820000 −0.112950000 N  1.296115000  0.832216000 −0.133430000 C  0.742210000  2.003067000 −0.019270000 C −0.742270000  2.003090000  0.019083000 N −1.296220000  0.832229000  0.133053000 N −2.641330000  0.681803000  0.112941000 C −3.023840000 −0.575820000 −0.062950000 S −1.997210000 −1.981990000 −0.398090000 N  4.355707000 −0.821760000  0.011951000 C  5.360703000  0.197081000 −0.248710000 C  1.515928000  3.286912000  0.086744000 C −1.515820000  3.287109000 −0.086270000 N −4.355720000 −0.821830000 −0.011720000 C −5.360710000  0.197241000  0.248053000 H  4.649104000 −1.769440000  0.197753000 H −4.649210000 −1.769530000 −0.197320000 H  6.342172000 −0.283450000 −0.243230000 H  5.337203000  0.979190000  0.518822000 H  5.196615000  0.669314000 −1.223320000 H  2.573561000  3.100014000 −0.103270000 H  1.415536000  3.723176000  1.090138000 H  1.146011000  4.031842000 −0.627510000 H −2.574120000  3.099479000  0.099275000 H −1.148810000  4.029876000  0.631789000 H −1.411500000  3.726523000 −1.087850000 H −6.342270000 −0.283090000  0.241886000 H −5.197320000  0.669605000  1.222733000 H −5.336430000  0.979236000 −0.519560000 H  3.281842792  1.442241206 −0.219812917 Zn(HL•) 0 2 Zn  0.000030000 −0.870350000 −0.000210000 S  1.997178000 −1.981870000  0.398596000 C  3.023857000 −0.575680000  0.063126000 N  2.641265000  0.681820000 −0.112950000 N  1.296115000  0.832216000 −0.133430000 C  0.742210000  2.003067000 −0.019270000 C −0.742270000  2.003090000  0.019083000 N −1.296220000  0.832229000  0.133053000 N −2.641330000  0.681803000  0.112941000 C −3.023840000 −0.575820000 −0.062950000 S −1.997210000 −1.981990000 −0.398090000 N  4.355707000 −0.821760000  0.011951000 C  5.360703000  0.197081000 −0.248710000 C  1.515928000  3.286912000  0.086744000 C −1.515820000  3.287109000 −0.086270000 N −4.355720000 −0.821830000 −0.011720000 C −5.360710000  0.197241000  0.248053000 H  4.649104000 −1.769440000  0.197753000 H −4.649210000 −1.769530000 −0.197320000 H  6.342172000 −0.283450000 −0.243230000 H  5.337203000  0.979190000  0.518822000 H  5.196615000  0.669314000 −1.223320000 H  2.573561000  3.100014000 −0.103270000 H  1.415536000  3.723176000  1.090138000 H  1.146011000  4.031842000 −0.627510000 H −2.574120000  3.099479000  0.099275000 H −1.148810000  4.029876000  0.631789000 H −1.411500000  3.726523000 −1.087850000 H −6.342270000 −0.283090000  0.241886000 H −5.197320000  0.669605000  1.222733000 H −5.336430000  0.979236000 −0.519560000 H  3.281842792  1.442241206 −0.219812917 [ZnH₂L•]⁺ 1 2 Zn  0.000030000 −0.870350000 −0.000210000 S  1.997178000 −1.981870000  0.398596000 C  3.023857000 −0.575680000  0.063126000 N  2.641265000  0.681820000 −0.112950000 N  1.296115000  0.832216000 −0.133430000 C  0.742210000  2.003067000 −0.019270000 C −0.742270000  2.003090000  0.019083000 N −1.296220000  0.832229000  0.133053000 N −2.641330000  0.681803000  0.112941000 C −3.023840000 −0.575820000 −0.062950000 S −1.997210000 −1.981990000 −0.398090000 N  4.355707000 −0.821760000  0.011951000 C  5.360703000  0.197081000 −0.248710000 C  1.515928000  3.286912000  0.086744000 C −1.515820000  3.287109000 −0.086270000 N −4.355720000 −0.821830000 −0.011720000 C −5.360710000  0.197241000  0.248053000 H  4.649104000 −1.769440000  0.197753000 H −4.649210000 −1.769530000 −0.197320000 H  6.342172000 −0.283450000 −0.243230000 H  5.337203000  0.979190000  0.518822000 H  5.196615000  0.669314000 −1.223320000 H  2.573561000  3.100014000 −0.103270000 H  1.415536000  3.723176000  1.090138000 H  1.146011000  4.031842000 −0.627510000 H −2.574120000  3.099479000  0.099275000 H −1.148810000  4.029876000  0.631789000 H −1.411500000  3.726523000 −1.087850000 H −6.342270000 −0.283090000  0.241886000 H −5.197320000  0.669605000  1.222733000 H −5.336430000  0.979236000 −0.519560000 H  3.281842792  1.442241206 −0.219812917 H −3.281933702  1.442187461  0.219910007 Protonation of ZnL at sulfur 1 1 30  0.000030000 −0.870350000 −0.000210000 16  1.997178000 −1.981870000  0.398596000 6  3.023857000 −0.575680000  0.063126000 7  2.641265000  0.681820000 −0.112950000 7  1.296115000  0.832216000 −0.133430000 6  0.742210000  2.003067000 −0.019270000 6 −0.742270000  2.003090000  0.019083000 7 −1.296220000  0.832229000  0.133053000 7 −2.641330000  0.681803000  0.112941000 6 −3.023840000 −0.575820000 −0.062950000 16 −1.997210000 −1.981990000 −0.398090000 7  4.355707000 −0.821760000  0.011951000 6  5.360703000  0.197081000 −0.248710000 6  1.515928000  3.286912000  0.086744000 6 −1.515820000  3.287109000 −0.086270000 7 −4.355720000 −0.821830000 −0.011720000 6 −5.360710000  0.197241000  0.248053000 1  4.649104000 −1.769440000  0.197753000 1 −4.649210000 −1.769530000 −0.197320000 1  6.342172000 −0.283450000 −0.243230000 1  5.337203000  0.979190000  0.518822000 1  5.196615000  0.669314000 −1.223320000 1  2.573561000  3.100014000 −0.103270000 1  1.415536000  3.723176000  1.090138000 1  1.146011000  4.031842000 −0.627510000 1 −2.574120000  3.099479000  0.099275000 1 −1.148810000  4.029876000  0.631789000 1 −1.411500000  3.726523000 −1.087850000 1 −6.342270000 −0.283090000  0.241886000 1 −5.197320000  0.669605000  1.222733000 1 −5.336430000  0.979236000 −0.519560000 1  2.274340046 −3.233237278  0.753787688 Protonation at Amine Nitrogen 1 1 Zn  0.000030000 −0.870350000 −0.000210000 S  1.997178000 −1.981870000  0.398596000 C  3.023857000 −0.575680000  0.063126000 N  2.641265000  0.681820000 −0.112950000 N  1.296115000  0.832216000 −0.133430000 C  0.742210000  2.003067000 −0.019270000 C −0.742270000  2.003090000  0.019083000 N −1.296220000  0.832229000  0.133053000 N −2.641330000  0.681803000  0.112941000 C −3.023840000 −0.575820000 −0.062950000 S −1.997210000 −1.981990000 −0.398090000 N  4.355707000 −0.821760000  0.011951000 C  5.360703000  0.197081000 −0.248710000 C  1.515928000  3.286912000  0.086744000 C −1.515820000  3.287109000 −0.086270000 N −4.355720000 −0.821830000 −0.011720000 C −5.360710000  0.197241000  0.248053000 H  4.649104000 −1.769440000  0.197753000 H −4.649210000 −1.769530000 −0.197320000 H  6.342172000 −0.283450000 −0.243230000 H  5.337203000  0.979190000  0.518822000 H  5.196615000  0.669314000 −1.223320000 H  2.573561000  3.100014000 −0.103270000 H  1.415536000  3.723176000  1.090138000 H  1.146011000  4.031842000 −0.627510000 H −2.574120000  3.099479000  0.099275000 H −1.148810000  4.029876000  0.631789000 H −1.411500000  3.726523000 −1.087850000 H −6.342270000 −0.283090000  0.241886000 H −5.197320000  0.669605000  1.222733000 H −5.336430000  0.979236000 −0.519560000 H  4.370833470 −0.021327902 −0.587281548 Protonation on Zinc 1 1 Zn  0.000030000 −0.870350000 −0.000210000 S  1.997178000 −1.981870000  0.398596000 C  3.023857000 −0.575680000  0.063126000 N  2.641265000  0.681820000 −0.112950000 N  1.296115000  0.832216000 −0.133430000 C  0.742210000  2.003067000 −0.019270000 C −0.742270000  2.003090000  0.019083000 N −1.296220000  0.832229000  0.133053000 N −2.641330000  0.681803000  0.112941000 C −3.023840000 −0.575820000 −0.062950000 S −1.997210000 −1.981990000 −0.398090000 N  4.355707000 −0.821760000  0.011951000 C  5.360703000  0.197081000 −0.248710000 C  1.515928000  3.286912000  0.086744000 C −1.515820000  3.287109000 −0.086270000 N −4.355720000 −0.821830000 −0.011720000 C −5.360710000  0.197241000  0.248053000 H  4.649104000 −1.769440000  0.197753000 H −4.649210000 −1.769530000 −0.197320000 H  6.342172000 −0.283450000 −0.243230000 H  5.337203000  0.979190000  0.518822000 H  5.196615000  0.669314000 −1.223320000 H  2.573561000  3.100014000 −0.103270000 H  1.415536000  3.723176000  1.090138000 H  1.146011000  4.031842000 −0.627510000 H −2.574120000  3.099479000  0.099275000 H −1.148810000  4.029876000  0.631789000 H −1.411500000  3.726523000 −1.087850000 H −6.342270000 −0.283090000  0.241886000 H −5.197320000  0.669605000  1.222733000 H −5.336430000  0.979236000 −0.519560000 H  0.000293000 −2.650349596 −0.001380325 Hydrogen 0 1 H 3.259348439 4.169555780 −0.124845483 H 3.259348439 4.169555780 −0.864845483

TABLE A7 Comparison of calculated energies for various protonation sites on ZnL Sum of Electronic and Thermal Site of Protonation Free Energies (kcal/mole) S-Protonation +1300 Hydrazino N-Protonation 0 Amine N-Protonation +13 Zn-Protonation +14

TABLE A8 α-β spin density comparison for radical species: Zn(HL•) and [ZnH₂L•]⁺ Spin-Density (α-β) Atom Zn(HL•) [ZnH₂L•]⁺ C1 0.096320 0.072671 C2 0.298443 0.211249 C3 — 0.211171 C4 0.094069 0.072688 N2 0.125707 0.192627 N3 0.259371 0.192651 N4 0.022704 — N5 — 0.033882 N6 0.026308 0.033891 S1 0.014559 — S2 0.008889 —

Example Set B HER of Cu Complexes

The compounds discussed in Example Set B include H₂L², ML¹, and ML².

Materials and Methods for Example Set B

Electrochemical Methods

All cyclic voltammetry (CV) and controlled potential coulometry (CPC) measurements were recorded using a Gamry Interface potentiostat/galvanostat, which was connected to a glassy carbon working electrode (6.5 mm diameter, surface =0.07 cm²), a platinum wire counter electrode, and Ag/AgCl reference electrode. Before use, the working electrode was polished using an aqueous alumina slurry. The working and counter electrodes were cleaned before use by washing with water, ethanol, isopropanol, acetone and then sonication for 10 minutes in acetonitrile. CV measurements were conducted using a three-neck electrochemical cell that was washed and dried in an oven overnight before use. All electrochemical experiments were conducted under a N₂ atmosphere. All CPC measurements were conducted using a two chambered glass electrolysis cell with working and auxiliary compartments separated by a frit, with a volume of 10 mL in each, washed and dried the night before use. The working compartment was fitted with a glassy carbon working electrode and an Ag/AgCl reference electrode. The auxiliary compartment was fitted with a Pt wire counter electrode. The working compartment contained 0.292 M acetic acid added to a 0.1 M Bu₄NPF₆ acetonitrile or DMF solution, while the auxiliary compartment was filled with 0.1 M Bu₄NPF₆ acetonitrile or DMF solution. Both compartments were purged for 15 min with N₂ prior to electrolysis. A control (blank) CPC study was conducted and subtracted from experimental results (supporting information). Electrolysis measurements were performed after addition of 0.6 mM CuL¹ to the working compartment for several time durations. The evolved gas was subjected to GC-TCD analysis at the end of the electrolysis using a Gow-Mac series 400 GC-TCD equipped with a molecular sieve column for product detection. The column was heated to 130° C. under N₂ gas flow with 250 μL injection samples injected onto the column to confirm H₂ as the gaseous product.

Overpotential Determination

Overpotential can be defined as the difference between the thermodynamic and equilibrium potentials for a given reaction and the potential at which the reaction occurs under a set of specific conditions. In the case of H₂ evolution or oxidation, when either the equilibrium potential for the standard state H⁺/H₂ couple (E°_(H+)) is not known for some particular solvent, or a reliable pKa scale is unavailable, the direct measurement of the equilibrium potential for the reduction of protons (E_(H+)) can be accomplished through an open circuit potential (OCP) measurements, as described by Appel and Helm (ACS Catal., 2014, Vol. 4, pp. 630-633; DOI: 10.1021/cs401013v). Using this method provides an accurate determination of the equilibrium potential for the H⁺/H₂ couple under a wide range of acids and bases, as well as solvents or mixtures of solvents. This method has proven valuable for the determination of E_(H+) of protic ionic liquids and various acid base pairs in acetonitrile DMF and/or water. The accurate determination of overpotential can sometimes require an estimation of E_(cat/2) and E_(H+), each of which can change, depending on the reaction conditions. The value for the potential for catalysis should be related to the catalytic current, and therefore, we use E_(cat/2). This combined with a value for E_(H+), obtained through OCP measurements allows for calculation of the overpotential (η) for proton reduction by CuL¹ under some specific experimental conditions. The overpotential can then be estimated as, η=|(E_(OCP)−E_(cat/2))|, where E_(OCP) is the measured open circuit potential measured under catalytic conditions specific for each reaction, and E_(cat/2) is the potential at one-half the maximum of the catalytic current measured for the catalyzed reduction of protons (see sample calculations).

Faradaic Efficiency Determination

Evolved gas from the cathode compartment displaced water in a cylinder with radius 1.12 cm by a height of 2.16 cm. Using the equation for the volume of a cylinder, V=π(r)²h, we can calculate the volume displaced. This is calculated to be 8.51 mL. Using the conversion factor of 22.4 L of any ideal gas per one mole of gas allows us to quantify the number of moles of H₂ evolved as 3.80+10⁴ moles. This value can then be compared to the theoretical number of moles of H₂ evolved based on charge determined earlier, 4.40×10⁴. Faradaic efficiency is defined as moles of H₂ quantified/moles of H₂ theoretical based on charge ×100%. This corresponds with a minimum Faradic efficiency of 86.0%.

Turnover Frequency Determination

i _(cat)=nFA[cat]√{square root over (Dk[H⁺]²)}  (B1)

Equation B1 details the relationship between the catalytic current i_(cat), the catalyst concentration [cat], and the acid concentration [H⁺] for a catalytic reaction that is second-order in acid and first-order in catalyst. The terms n, F, A, and D are the normal electrochemical terms related to the number of electrons transferred, Faraday's constant, area of the electrode (0.07 cm²), and diffusion constant, respectively.

Equation B2 (Randle-Sevcik equation) provides the relationship between the peak current (i_(p)), catalyst concentration, and scan rate (v) in the absence of acid. The factor of 0.4463 is related to the diffusion equations, R is the gas constant, and T is temperature in K. The other terms are the same as in equation B 1.

$\begin{matrix} {i_{p} = {0.4463{{FA}\lbrack{cat}\rbrack}\sqrt{\frac{FvD}{RT}}}} & ({B2}) \end{matrix}$

Thus, the ratio of i_(cat)/i_(p) (equation B3) is obtained from the quotient equations B1 and B2.

$\begin{matrix} {\frac{i_{cat}}{i_{p}} = {\frac{n}{0.4463}\sqrt{\frac{{{RTk}\left\lbrack H^{+} \right\rbrack}^{2}}{Fv}}}} & ({B3}) \end{matrix}$

Under pseudo first-order conditions where k_(obs)=k[H⁺]², equation B3 simplifies to B4.

$\begin{matrix} {\frac{i_{cat}}{i_{p}} = {\frac{n}{0.4463}\sqrt{\frac{{RTk}_{obs}}{Fv}}}} & ({B4}) \end{matrix}$

Equation B4 can be simplified further to equation B5, when n=2, and when at scan rate independent conditions can be used to estimate the observed rate constant or turnover frequency (TOF) (see sample calculations).

k _(obs)=1.94×v[i _(cat) /i _(p)]²   (B5)

X-ray Photoelectron Spectroscopy of Electrode Adsorbed Films

CPEs of 0.6 mM CuL¹ with 0.292 M acetic acid added in 0.1 M Bu₄NPF₆ DMF and ACN solutions were run for 23.5 and 4.2 hours, respectively. After completion of electrolysis, the working electrode was removed and washed with DI water. A visible red-brown film persisted on the electrode surface. The films were scraped off using a spatula, collected onto wax paper, and transferred to a glass vial, which was sealed and wrapped with parafilm. XPS analysis was conducted by the Nanoscale Characterization Facility at the University of Indiana (Bloomington, Ind.) using a PHI VersaProbe II Scanning X-ray Microprobe system.

Computational Methods

Initial calculations were performed using M06, B3LYP and B97-D. Based on energetic minima results, B3LYP was chosen for use as the functional for subsequent calculations. Optimizations were performed in the gas phase using density functional theory (DFT) employing the B3LYP exchange correlation functional, and the 6-311G(d,p) basis set for all atoms as implemented in the Gaussian09 suite of programs for electronic structure and ChemCraft was used for graphics visualization. All optimizations were performed under tight constraints, with no symmetry imposed. All input coordinates are available below.

Sample Calculations

Overpotential calculation at the potential of half catalytic current (E_(cat/2)); CuL¹ HER:

η=Overpotential=|(E _(BH+(OCP)))−(E _(cat/2))|

η=|[−0.50−(−2.20 V)]|

η=1.7 V vs Fc⁺/Fc⁰

Determination of CuL¹ Diffusion Coefficient (D₀) (Acetonitrile):

Slope=3.22E−5=0.4463FA[cat][(FD₀/RT)]^(0.5)

-   -   A=0.071 cm²     -   [cat]=6E-7 moles/cm³     -   F=96485 C/mole e⁻     -   R=ideal gas constant     -   T=298 K         -   D₀=7.9E−6 cm²/s in acetonitrile

Determination of CuL¹ Diffusion Coefficient (D₀) (DMF):

Slope=3E−5=0.4463FA[cat][(FD₀/RT)]^(0.5)

-   -   A=0.071 cm²     -   [cat]=6E−7 moles/cm³     -   F=96485 C/mole e⁻     -   R=ideal gas constant     -   T=298 K         -   D₀=9.35E−6 cm²/s in DMF

TOF_(max) Sample Calculation for CuL¹ in acetonitrile:

Using equation B5, at scan-rate independent conditions, when v=0.2 v/s and when i_(cat)=2250 μA and i_(p)=14 μA

$\frac{i_{cat}}{i_{p}} = 160.71$ TOF_(max)/k_(obs) = 10000  s⁻¹

TOF_(max) Sample Calculation for CuL¹ in Dimethylformamide:

Using equation B5, at scan rate independent conditions, when v=1.0 v/s, and when i_(cat)=1490 μA and i_(p)=29 μA

$\frac{i_{cat}}{i_{p}} = 51.44$ TOF_(max)/k_(obs) = 5410  s⁻¹

Sample Calculations CuL¹ Electrolysis in Acetonitrile:

Trial 1: Total charge=Q_(with cat)−Q_(blank)=Q_(net)

60.49 C−0.0576 C=60.43

Theoretical Moles of Hydrogen Made via Total Charge:

60.43 C×(1 mol e ⁻/96485 C)×(1 mol H₂/2 mol e ⁻)=moles H₂ theoretical

-   -   Moles H₂ theoretical=0.00031 moles H₂ based on charge from         electrolysis

Trial 1: CuL¹ TON Calculation:

TON=Moles of H₂ Produced/Moles of CuL¹ Used

TON=(0.00031 moles H₂ produced)/(0.000006 moles CuL¹ used)

-   -   TON=51.7

Trial 2: Total Charge=Q_(with cat)−Q_(blank)=Q_(net)

84.74 C−0.0576 C=84.68

Theoretical moles of Hydrogen made via Total Charge

84.68 C×(1 mol e ⁻/96485 C)×(1 mol H₂/2 mol e ⁻)=moles H₂ theoretical

-   -   Moles H₂ Theoretical=0.00044 moles H₂ based on charge from         electrolysis

Trial 2: CuL¹ TON Calculation:

TON=Moles of H₂ Produced/Moles of CuL¹ Used

TON=(0.00044 moles of H₂ produced)/(0.000006 moles CuL¹ used)

-   -   TON=73.3

Sample Calculations CuL¹ Electrolysis in DMF

Trial 1: Total charge=Q_(with cat)−Q_(blank)=Q_(net)

67.03 C−0.0682 C=66.96 C

Theoretical Moles of Hydrogen made via total Charge:

66.96×(1 mol e ⁻/96485 C)×(1 mol H₂/2 mol e ⁻)=moles of H₂ theoretical

-   -   Moles H₂ Theoretical=0.00035 moles H₂ based on charge from         electrolysis

Trial 1: CuL¹ TON Calculation:

TON=Moles of H₂ Produced/Moles of CuL¹ Used

TON=(0.00035 moles of H₂ produced)/(0.000006 moles CuL¹ used)

-   -   TON=58.3

Trial 2: Total Charge=Q_(with cat)−Q_(blank)=Q_(net)

85.06 C−0.682 C=85 C

Theoretical Moles of Hydrogen made via total Charge:

85×(1 mol e ⁻/96485 C)×(1 mol H₂/2 mol e ⁻)=moles of H₂ theoretical

-   -   Moles H₂ Theoretical=0.00044 moles H₂ based on charge from         electrolysis

Trial 2: CuL¹ TON Calculation:

TON=Moles of H₂ Produced/Moles of CuL¹ Used

TON=(0.00044 moles of H₂ produced)/(0.000006 moles CuL¹ used)

-   -   TON=73.3

Trial 2: CuL¹ Faradaic Efficiency Calculation:

Faradaic  efficiency = (moles  of  H₂  quantified)/(moles  of  H₂  theoretical  based  on  charge) × 100% = (0.000356moles)/(0.00044  Moles) × 100% = 81%  Faradaic  Efficiency

Crystallographic Details

A light-purple plate 0.26×0.10×0.01 mm³ crystal of [CuL¹H_(2]) ²⁺, grown through liquid-liquid diffusion of pentane into methanol/acetonitrile solution of 1 mM CuL¹ with four drops of perchloric acid added, was mounted on a CryoLoop for collection of x-ray data on an Agilent Technologies/Oxford Diffraction Gemini CCD diffractometer. The CrysAlisPro¹ CCD software package (v 1.171.36.32) was used to acquire a total of 772 forty-five second frame ω-scan exposures of data at 100K to a 2θ max=57.42° using monochromated MoKα radiation (0.71073 Å) from a sealed tube. Frame data were processed using CrysAlisPro¹ RED to determine final unit cell parameters: a=8.7724(3) Å, b=9.3218(3) Å, c=12.1476(5) Å, α=100.149(3), β=107.682(3)°, γ=97.493(3)°, V=913.55(6) Å³, D_(calc)=1.901 Mg/m³, Z=2 to produce raw hkl data that were then corrected for absorption (transmission min./max.=0.848/1.000; μ=1.769 mm⁻¹) using SCALE3 ABSPACK. The structure was solved by Direct methods in the space group P-1 using SHELXS and refined by least squares methods on F² using SHELXL. Non-hydrogen atoms were refined with anisotropic atomic displacement parameters. Imine H's were located by difference maps and refined isotropically. Methyl hydrogen atoms were placed in their geometrically generated positions and refined as a riding model and these atoms were assigned U(H)=1.5×Ueq. For all 4720 unique reflections (R(int) 0.040) the final anisotropic full matrix least-squares refinement on F² for 264 variables converged at R1=0.044 and wR2=0.075 with a GOF of 1.06.

TABLE B1 Bond lengths (Å) for [Cu(L¹H₂)(ClO₄)]ClO_(4•) Cu(1)—N(1) 1.9579(18) C(2)—C(4) 1.483(3) Cu(1)—N(4) 1.9557(18) C(3)—H(3A) 0.9600 Cu(1)—S(1) 2.2462(6) C(3)—H(3B) 0.9600 Cu(1)—S(2) 2.2593(6) C(3)—H(3C) 0.9600 S(1)—C(5) 1.714(2) C(4)—H(4A) 0.9600 S(2)—C(6) 1.711(2) C(4)—H(4B) 0.9600 N(1)—C(1) 1.282(3) C(4)—H(4C) 0.9600 N(1)—N(2) 1.355(3) C(7)—H(7A) 0.9600 N(2)—C(5) 1.360(3) C(7)—H(7B) 0.9600 N(2)—H(2N) 0.78(3) C(7)—H(7C) 0.9600 N(3)—C(5) 1.311(3) C(8)—H(8A) 0.9600 N(3)—C(7) 1.448(3) C(8)—H(8B) 0.9600 N(3)—H(3N) 0.76(3) C(8)—H(8C) 0.9600 N(4)—C(2) 1.287(3) Cl(1)—O(2) 1.4223(17) N(4)—N(5) 1.364(2) Cl(1)—O(4) 1.4274(18) N(5)—C(6) 1.356(3) Cl(1)—O(3) 1.4503(17) N(5)—H(5N) 0.78(2) Cl(1)—O(1) 1.4541(17) N(6)—C(6) 1.319(3) Cl(2)—O(8) 1.4231(19) N(6)—C(8) 1.465(3) Cl(2)—O(7) 1.4307(19) N(6)—H(6N) 0.78(3) Cl(2)—O(6) 1.430(2) C(1)—C(3) 1.484(3) Cl(2)—O(5) 1.4506(18) C(1)—C(2) 1.501(3)

TABLE B2 Bond angles (°) for [Cu(L¹H₂)(ClO₄)]ClO_(4•) N(1)—Cu(1)—N(4) 78.80(7) C(2)—C(4)—H(4A) 109.5 N(1)—Cu(1)—S(1) 86.88(6) C(2)—C(4)—H(4B) 109.5 N(4)—Cu(1)—S(1) 165.50(5) H(4A)—C(4)—H(4B) 109.5 N(1)—Cu(1)—S(2) 164.38(6) C(2)—C(4)—H(4C) 109.5 N(4)—Cu(1)—S(2) 86.08(5) H(4A)—C(4)—H(4C) 109.5 S(1)—Cu(1)—S(2) 108.02(2) H(4B)—C(4)—H(4C) 109.5 C(5)—S(1)—Cu(1) 95.64(8) N(3)—C(5)—N(2) 116.5(2) C(6)—S(2)—Cu(1) 95.99(7) N(3)—C(5)—S(1) 121.83(19) C(1)—N(1)—N(2) 123.56(18) N(2)—C(5)—S(1) 121.63(17) C(1)—N(1)—Cu(1) 118.13(15) N(6)—C(6)—N(5) 117.3(2) N(2)—N(1)—Cu(1) 118.19(14) N(6)—C(6)—S(2) 120.89(17) N(1)—N(2)—C(5) 117.61(19) N(5)—C(6)—S(2) 121.84(16) N(1)—N(2)—H(2N) 121(2) N(3)—C(7)—H(7A) 109.5 C(5)—N(2)—H(2N) 121(2) N(3)—C(7)—H(7B) 109.5 C(5)—N(3)—C(7) 124.8(2) H(7A)—C(7)—H(7B) 109.5 C(5)—N(3)—H(3N) 119(2) N(3)—C(7)—H(7C) 109.5 C(7)—N(3)—H(3N) 116(2) H(7A)—C(7)—H(7C) 109.5 C(2)—N(4)—N(5) 122.83(19) H(7B)—C(7)—H(7C) 109.5 C(2)—N(4)—Cu(1) 118.03(15) N(6)—C(8)—H(8A) 109.5 N(5)—N(4)—Cu(1) 119.13(14) N(6)—C(8)—H(8B) 109.5 C(6)—N(5)—N(4) 116.86(18) H(8A)—C(8)—H(8B) 109.5 C(6)—N(5)—H(5N) 120.5(19) N(6)—C(8)—H(8C) 109.5 N(4)—N(5)—H(5N) 119.9(19) H(8A)—C(8)—H(8C) 109.5 C(6)—N(6)—C(8) 123.5(2) H(8B)—C(8)—H(8C) 109.5 C(6)—N(6)—H(6N) 117(2) O(2)—C1(1)—O(4) 110.85(12) C(8)—N(6)—H(6N) 119(2) O(2)—C1(1)—O(3) 109.38(11) N(1)—C(1)—C(3) 125.2(2) O(4)—C1(1)—O(3) 110.25(11) N(1)—C(1)—C(2) 112.56(18) O(2)—C1(1)—O(1) 110.29(11) C(3)—C(1)—C(2) 122.2(2) O(4)—C1(1)—O(1) 109.21(10) N(4)—C(2)—C(4) 124.6(2) O(3)—C1(1)—O(1) 106.78(10) N(4)—C(2)—C(1) 112.47(19) O(8)—C1(2)—O(7) 109.28(13) C(4)—C(2)—C(1) 122.83(19) O(8)—C1(2)—O(6) 110.09(14) C(1)—C(3)—H(3A) 109.5 O(7)—C1(2)—O(6) 109.35(12) C(1)—C(3)—H(3B) 109.5 O(8)—C1(2)—O(5) 109.98(11) H(3A)—C(3)—H(3B) 109.5 O(7)—C1(2)—O(5) 109.47(12) C(1)—C(3)—H(3C) 109.5 O(6)—C1(2)—O(5) 108.65(11) H(3A)—C(3)—H(3C) 109.5 H(3B)—C(3)—H(3C) 109.5

TABLE B3 Kinetic Isotope Effect: CH₃COOH vs CD₃COOD % D-Acid i_(cat) (uA) TOF (s⁻¹) KIE 0.00 2250 10021.68 1 20.00 1837 6680.269 1.500192 40.00 1257 3127.852 3.204015 60.00 1045 2161.764 4.635883 80.00 910 1639.3 6.113392 100.00 819 1327.833 7.547398

TABLE B4 Computational Input Coordinates CuL¹ 0 2 Cu  1.431847450 −0.488032580  0.446112510 S  2.332615610 −0.119942340  2.552696700 S  2.858402730 −1.722415690 −0.909505780 N −0.257435500  0.971116720  2.468152050 N −0.172027540  0.534274340  1.177021620 N  0.574079220 −1.034389140 −2.391256330 N  0.223704470 −0.461096440 −1.204115870 C  0.866804360  0.696900310  3.160702940 C −1.156654600  0.726779040  0.313291070 C  1.780157800 −1.630949380 −2.329823970 C −0.928456430  0.172470660 −1.041194200 C −2.430852170  1.445672040  0.707410510 C −1.949721220  0.325466600 −2.151313540 N  0.905903760  1.099112750  4.470338340 N  2.249331710 −2.229486700 −3.469048500 C  1.542073250 −2.300940750 −4.757663730 C −0.256906930  1.621968390  5.210058210 H −0.736089590  2.427871470  4.633939270 H  0.095862870  2.020878310  6.174487110 H −1.009760380  0.833110030  5.396764760 H −3.052880760  1.693994160 −0.164430810 H −2.187359810  2.369822740  1.257044200 H −3.027972700  0.817502200  1.394048690 H  2.232241150 −2.034661770 −5.577035900 H  0.709065320 −1.588742850 −4.742827890 H  1.143750430 −3.316026930 −4.940576080 H −1.595749020 −0.186808630 −3.055724620 H −2.117801900  1.390722990 −2.392856600 H −2.924379830 −0.103170090 −1.856998090 H  1.714229460  0.779113230  5.003296850 H  3.143979070 −2.710111620 −3.380991940 [CuL¹H]⁺ 1 2 Cu  1.431847450 −0.488032580  0.446112510 S  2.332615610 −0.119942340  2.552696700 S  2.858402730 −1.722415690 −0.909505780 N −0.257435500  0.971116720  2.468152050 N −0.172027540  0.534274340  1.177021620 N  0.574079220 −1.034389140 −2.391256330 N  0.223704470 −0.461096440 −1.204115870 C  0.866804360  0.696900310  3.160702940 C −1.156654600  0.726779040  0.313291070 C  1.780157800 −1.630949380 −2.329823970 C −0.928456430  0.172470660 −1.041194200 C −2.430852170  1.445672040  0.707410510 C −1.949721220  0.325466600 −2.151313540 N  0.905903760  1.099112750  4.470338340 N  2.249331710 −2.229486700 −3.469048500 C  1.542073250 −2.300940750 −4.757663730 C −0.256906930  1.621968390  5.210058210 H −0.736089590  2.427871470  4.633939270 H  0.095862870  2.020878310  6.174487110 H −1.009760380  0.833110030  5.396764760 H −3.052880760  1.693994160 −0.164430810 H −2.187359810  2.369822740  1.257044200 H −3.027972700  0.817502200  1.394048690 H  2.232241150 −2.034661770 −5.577035900 H  0.709065320 −1.588742850 −4.742827890 H  1.143750430 −3.316026930 −4.940576080 H −1.595749020 −0.186808630 −3.055724620 H −2.117801900  1.390722990 −2.392856600 H −2.924379830 −0.103170090 −1.856998090 H  1.714229460  0.779113230  5.003296850 H  3.143979070 −2.710111620 −3.380991940 H  0.002279216 −1.014134076 −3.211399298 CuL¹H 0 1 Cu  1.431847450 −0.488032580  0.446112510 S  2.332615610 −0.119942340  2.552696700 S  2.858402730 −1.722415690 −0.909505780 N −0.257435500  0.971116720  2.468152050 N −0.172027540  0.534274340  1.177021620 N  0.574079220 −1.034389140 −2.391256330 N  0.223704470 −0.461096440 −1.204115870 C  0.866804360  0.696900310  3.160702940 C −1.156654600  0.726779040  0.313291070 C  1.780157800 −1.630949380 −2.329823970 C −0.928456430  0.172470660 −1.041194200 C −2.430852170  1.445672040  0.707410510 C −1.949721220  0.325466600 −2.151313540 N  0.905903760  1.099112750  4.470338340 N  2.249331710 −2.229486700 −3.469048500 C  1.542073250 −2.300940750 −4.757663730 C −0.256906930  1.621968390  5.210058210 H −0.736089590  2.427871470  4.633939270 H  0.095862870  2.020878310  6.174487110 H −1.009760380  0.833110030  5.396764760 H −3.052880760  1.693994160 −0.164430810 H −2.187359810  2.369822740  1.257044200 H −3.027972700  0.817502200  1.394048690 H  2.232241150 −2.034661770 −5.577035900 H  0.709065320 −1.588742850 −4.742827890 H  1.143750430 −3.316026930 −4.940576080 H −1.595749020 −0.186808630 −3.055724620 H −2.117801900  1.390722990 −2.392856600 H −2.924379830 −0.103170090 −1.856998090 H  1.714229460  0.779113230  5.003296850 H  3.143979070 −2.710111620 −3.380991940 H  0.002279216 −1.014134076 −3.211399298 CuL¹H (Triplet) 0 3 Cu  1.431847450 −0.488032580  0.446112510 S  2.332615610 −0.119942340  2.552696700 S  2.858402730 −1.722415690 −0.909505780 N −0.257435500  0.971116720  2.468152050 N −0.172027540  0.534274340  1.177021620 N  0.574079220 −1.034389140 −2.391256330 N  0.223704470 −0.461096440 −1.204115870 C  0.866804360  0.696900310  3.160702940 C −1.156654600  0.726779040  0.313291070 C  1.780157800 −1.630949380 −2.329823970 C −0.928456430  0.172470660 −1.041194200 C −2.430852170  1.445672040  0.707410510 C −1.949721220  0.325466600 −2.151313540 N  0.905903760  1.099112750  4.470338340 N  2.249331710 −2.229486700 −3.469048500 C  1.542073250 −2.300940750 −4.757663730 C −0.256906930  1.621968390  5.210058210 H −0.736089590  2.427871470  4.633939270 H  0.095862870  2.020878310  6.174487110 H −1.009760380  0.833110030  5.396764760 H −3.052880760  1.693994160 −0.164430810 H −2.187359810  2.369822740  1.257044200 H −3.027972700  0.817502200  1.394048690 H  2.232241150 −2.034661770 −5.577035900 H  0.709065320 −1.588742850 −4.742827890 H  1.143750430 −3.316026930 −4.940576080 H −1.595749020 −0.186808630 −3.055724620 H −2.117801900  1.390722990 −2.392856600 H −2.924379830 −0.103170090 −1.856998090 H  1.714229460  0.779113230  5.003296850 H  3.143979070 −2.710111620 −3.380991940 H  0.002279216 −1.014134076 −3.211399298 [CuL¹H₂]⁺ 1 1 Cu  1.201842280 −0.715405322  0.506889342 S  2.102610440 −0.347315082  2.613473532 S  2.628397560 −1.949788432 −0.848728948 N −0.487440670  0.743743978  2.528928882 N −0.402032710  0.306901598  1.237798452 N  0.344074050 −1.261761882 −2.330479498 N −0.006300700 −0.688469182 −1.143339038 C  0.636799190  0.469527568  3.221479772 C −1.386659770  0.499406298  0.374067902 C  1.550152630 −1.858322122 −2.269047138 C −1.158461600 −0.054902082 −0.980417368 C −2.660857340  1.218299298  0.768187342 C −2.179726390  0.098093858 −2.090536708 N  0.675898590  0.871740008  4.531115172 N  2.019326540 −2.456859442 −3.408271668 C  1.312068080 −2.528313492 −4.696886898 C −0.486912100  1.394595648  5.270835042 H −0.966094760  2.200498728  4.694716102 H −0.134142300  1.793505568  6.235263942 H −1.239765550  0.605737288  5.457541592 H −3.282885930  1.466621418 −0.103653978 H −2.417364980  2.142449998  1.317821032 H −3.257977870  0.590129458  1.454825522 H  2.002235980 −2.262034512 −5.516259068 H  0.479060150 −1.816115592 −4.682051058 H  0.913745260 −3.543399672 −4.879799248 H −1.825754190 −0.414181372 −2.994947788 H −2.347807070  1.163350248 −2.332079768 H −3.154385000 −0.330542832 −1.796221258 H  1.484224290  0.551740488  5.064073682 H  2.913973900 −2.937484362 −3.320215108 H −0.227725954 −1.241506818 −3.150622466 H −1.284747732  1.209226221  2.913152425 CuL•¹H₂—N1H 0 2 Cu  1.201842280 −0.715405322  0.506889342 S  2.102610440 −0.347315082  2.613473532 S  2.628397560 −1.949788432 −0.848728948 N −0.487440670  0.743743978  2.528928882 N −0.402032710  0.306901598  1.237798452 N  0.344074050 −1.261761882 −2.330479498 N −0.006300700 −0.688469182 −1.143339038 C  0.636799190  0.469527568  3.221479772 C −1.386659770  0.499406298  0.374067902 C  1.550152630 −1.858322122 −2.269047138 C −1.158461600 −0.054902082 −0.980417368 C −2.660857340  1.218299298  0.768187342 C −2.179726390  0.098093858 −2.090536708 N  0.675898590  0.871740008  4.531115172 N  2.019326540 −2.456859442 −3.408271668 C  1.312068080 −2.528313492 −4.696886898 C −0.486912100  1.394595648  5.270835042 H −0.966094760  2.200498728  4.694716102 H −0.134142300  1.793505568  6.235263942 H −1.239765550  0.605737288  5.457541592 H −3.282885930  1.466621418 −0.103653978 H −2.417364980  2.142449998  1.317821032 H −3.257977870  0.590129458  1.454825522 H  2.002235980 −2.262034512 −5.516259068 H  0.479060150 −1.816115592 −4.682051058 H  0.913745260 −3.543399672 −4.879799248 H −1.825754190 −0.414181372 −2.994947788 H −2.347807070  1.163350248 −2.332079768 H −3.154385000 −0.330542832 −1.796221258 H  1.484224290  0.551740488  5.064073682 H  2.913973900 −2.937484362 −3.320215108 H −0.227725954 −1.241506818 −3.150622466 H −1.284747732  1.209226221  2.913152425 CuL•¹H₂—N4H 0 2 Cu  1.431847450 −0.488032580  0.446112510 S  2.332615610 −0.119942340  2.552696700 S  2.858402730 −1.722415690 −0.909505780 N −0.257435500  0.971116720  2.468152050 N −0.172027540  0.534274340  1.177021620 N  0.574079220 −1.034389140 −2.391256330 N  0.223704470 −0.461096440 −1.204115870 C  0.866804360  0.696900310  3.160702940 C −1.156654600  0.726779040  0.313291070 C  1.780157800 −1.630949380 −2.329823970 C −0.928456430  0.172470660 −1.041194200 C −2.430852170  1.445672040  0.707410510 C −1.949721220  0.325466600 −2.151313540 N  0.905903760  1.099112750  4.470338340 N  2.249331710 −2.229486700 −3.469048500 C  1.542073250 −2.300940750 −4.757663730 C −0.256906930  1.621968390  5.210058210 H −0.736089590  2.427871470  4.633939270 H  0.095862870  2.020878310  6.174487110 H −1.009760380  0.833110030  5.396764760 H −3.052880760  1.693994160 −0.164430810 H −2.187359810  2.369822740  1.257044200 H −3.027972700  0.817502200  1.394048690 H  2.232241150 −2.034661770 −5.577035900 H  0.709065320 −1.588742850 −4.742827890 H  1.143750430 −3.316026930 −4.940576080 H −1.595749020 −0.186808630 −3.055724620 H −2.117801900  1.390722990 −2.392856600 H −2.924379830 −0.103170090 −1.856998090 H  1.714229460  0.779113230  5.003296850 H  3.143979070 −2.710111620 −3.380991940 H  0.002279216 −1.014134076 −3.211399298 H −0.537578235 −0.345922291 −1.882139314

Results and Discussion for Example B

Synthesis and Electrochemical Characterization

The CuL¹ compound was isolated as an air-stable burgundy solid from H₂L¹ and copper(II) acetate as previously reported previously (Betts et al., Angew. Chem. Int. Ed. 2008, Vol. 44, pp. 8416-8419—DOI: 10.1002/anie.200801936; Christlieb et al., Dalton Trans. 2007, pp. 5043-5054—DOI: 10.1039/B705087A). The cyclic voltammogram (CV) of CuL¹ in acetonitrile (ACN) or dimethylformamide (DMF) containing 0.1 M Bu₄NPF₆ as supporting electrolyte displays a reversible Cu^(II/I) event at −1.20 V vs. ferrocenium/ferrocene (Fc⁺/Fc) consistent with prior reports. Additional CV data collected at multiple scan rates from 0.1 to 1.0 V/s in ACN (FIG. 24) and DMF (data not shown) were used to construct Cottrell plots (FIG. 24 inset) establishing that the Cu^(II/I) reduction is diffusion limited and demonstrating the potential of CuL¹ as a homogeneous electrocatalyst. The slope of the plot yields a diffusion coefficient of 7.9×10⁻⁶ cm²/s in ACN and 9.35×10⁻⁶ cm²/s in DMF. The formal Cu^(III/II) couple was observed at 0.24 V vs Fc⁺/Fc in DMF and ACN in line with prior reports, but this event was not further evaluated in the current study.

Homogeneous Catalytic Hydrogen Evolution: Cyclic Voltammetry and KIE

Addition of acetic acid exceeding 24 mM to 0.6 mM ACN solutions of CuL¹ shifts the Cu^(II/I) reduction potential from −1.20 V to −0.95 V and introduces a catalytic cathodic current at −1.70 V vs Fc⁺/Fc (FIG. 25A). The +0.25 V shift is consistent with a single protonation event prior to the initial electrochemical reduction. The ratio of the catalytic current to the peak current displays linear dependence on the acid concentration up to 0.157 M, indicating a second-order dependence of the catalytic rate on the acid concentration. At concentrations greater than 0.157 M the current response begins to plateau, reaching an acid independent region at concentrations of 0.269 M, (FIG. 25B). This transition from second-order to zero-order dependence requires a pre-equilibrium step(s) involving two protons that precedes the rate determining step for H₂ elimination. The current becomes scan rate independent at 0.2 V/s. Under these conditions the i_(p) of the Cu^(II/I) reduction event, 14.0 μA, and the i_(cat) max from the acid-independent region, 2.25 mA, correspond with a maximum i_(cat)/i_(p) value of 161, affording a TOF of 10,000 s⁻¹.

The electrocatalytic activity of 0.6 mM CuL¹ with acetic acid was also assessed in DMF. An increase in current at −1.9 V vs Fc⁺/Fc is observed upon increasing additions of acetic acid (FIG. 26A). At concentrations of acid greater than 0.292 M, the current saturates reaching a maximum cat of 1.49 mA (FIG. 26B). Acid addition results in a shift of the Cu^(II/I) potential from −1.20 V to −0.95 V, as observed in ACN, attributed to a single protonation event prior to reduction. Catalytic current becomes independent of scan rate above 1.0 V/s (FIG. 26C and 26D). Under these conditions i_(cat) is 1490 μA and i_(p) is 29 μA giving a TOF of 5140 s⁻¹, which is significantly lower than the TOF in ACN.

The Tafel plot of the log TOF versus overpotential for CuL¹ (FIG. 25C) displays the TOF as a function of the applied overpotential. CuL¹ displays the highest maximum logTOF values reported to date of any homogeneous ligand-centered electrocatalyst, reaching a maximum of 3.99. The CuL¹ electrocatalyst maintains a logTOF value greater than one, with applied overpotentials greater than 1.2 V. Overpotentials less than 1.2 V result in significantly decreased TOF values, correlating with negative or near zero logTOF values. The local maxima observed near 0.4 V is indicative of the pre-catalytic Cu^(II/I) reduction, which has anodic shift of 0.25 V during catalysis. The maximum logTOF for CuL¹ of 3.99 requires a large overpotential of 2.0 V.

Analysis of the CVs of CuL¹ under catalytic HER conditions appear to reveal a new oxidation event at a potential of −0.65 V Fc⁺/Fc during the return anodic scan (FIG. 25A inset), which is assigned to the Cu(II/I) couple of the diprotonated copper(I) intermediate, [CuL¹H₂]⁺. This event is 300 mV more positive than the Cu(II/I) couple of the monoprotonated [CuL¹H]⁺, which in turn is 250 mV more positive than the CuL¹. The intensity of the peak current at −0.65 V demonstrates scan rate dependence typical of diffusion controlled behavior (FIG. 26C) confirming it is not due to an adsorbed species.

As noted above, the catalysis is second-order in acid in the acid dependent regime. To determine the order with respect to the catalyst, the concentration of CuL¹ was varied from 0.1 to 1.0 mM in solution containing 0.15 M acetic acid. A plot of catalyst concentration versus peak current reveals a linear relationship, confirming a first-order dependence on the concentration of the catalyst and an overall third-order process.

To further evaluate the HER mechanism of CuL¹, the H/D kinetic isotope effect (KIE) was measured. CuL¹ displays a large KIE of 7.54 using 100% CD₃CO₂D. The high KIE value observed when using 100% CD₃CO₂D is distinct from the inverse KIEs reported for some HER catalysts proceeding through metal-hydrides, but similar to that observed for a ligand-centered Re-thiolate HER catalyst. Since CuL¹ HER catalysis is second-order in [H⁺], a proton inventory study was conducted to determine the number of protons involved in the rate determining step. Cyclic voltammograms collected with variable quantities of CH₃CO₂H and CD₃CO₂D were used to generate plots of KIE versus the percent fraction of CD₃CO₂D (FIGS. 27A & 27B). The plot yields a linear fit consistent with the involvement of a single proton in the rate determining step.

Controlled Potential Electrolysis

A series of controlled potential electrolysis (CPE) experiments were performed using 0.6 mM CuL¹ and 0.292 M acetic acid, with potential held at −1.65 V vs Fc⁺/Fc, in both DMF and ACN (Table B5). Electrolysis in 0.1 M Bu₄NPF₆ DMF solutions was allowed to run for 84,400 seconds (23.4 hours) resulting in a total charge passed of 85.0 C, corresponding to 4.4×10⁻⁴ moles of H₂ produced with a turnover number (TON) of 73.3. Gas analysis of the headspace using gas chromatography thermal conductivity (GC-TCD) confirms H₂ as the gaseous product (FIG. 41). The charge increases linearly over time with no signs of degradation or decrease in activity over 23 hours (FIG. 28). A second CPE in DMF over 72,120 seconds (20 hours) yielded comparable results giving a slightly lower charge of 67.0 C, producing 3.5×10⁻⁴ moles of H₂ corresponding with a TON of 58.3.

TABLE B5 Summary of CPE Results Duration Charge Moles of H₂ Entry Solvent (S⁻¹) ( C) Produced (×10⁻⁴) TON 1 ACN 15,000 60.4 3.1 52 2 ACN 13,000 84.7 4.4 73 3 DMF 84,400 67.0 3.5 58 4 DMF 72,120 85.0 4.4 73 The CPEs performed in 0.1 M Bu₄NPF₆ ACN passed similar charge, giving values of 60.4 and 84.7 C corresponding to TON values of 51.7 and 73.3, over shorter electrolysis times of 15,000 and 13,000 seconds, respectively (FIG. 28). The current in ACN is higher than in DMF, resulting in a steeper slope in the charge-time plots, consistent with relative TOFs from CV studies. Electrolysis beyond 15,000 seconds in ACN is complicated by diffusion across the frit from the working to auxiliary compartment. This results in the appearance of a brown, cloudy mixture in the auxiliary compartment concurrent with the plateauing of charge. This phenomenon was consistently observed in ACN, but was absent in DMF.

Control Experiments

A series of control experiments were performed to confirm CuL¹ as the electrocatalyst. First, CVs were recorded on ACN and DMF solutions containing only acetic acid. Addition of 67.2 mM acetic acid to 0.1 M Bu₄NPF₆ ACN solutions resulted in an observable current of 300 μA (FIG. 29A). However, after 2 CV cycles the current drops to a stable value near 100 μA. Upon addition of 0.6 mM CuL¹, the current increases to 900 μA (FIG. 29A). In DMF addition of 22.4 mM acetic acid results in a modest current increase of ˜5 μA (FIG. 29B). Addition of 0.6 mM CuL¹ to this solution resulted in an increase in current, giving a value of 200 μA (FIG. 29B). At more cathodic potentials, a substantial change in the CVs of CuL¹ with 22.4 mM acetic acid added is observed with current increase onset potentials near −1.7 V vs Fc⁺/Fc (FIG. 29B). These control experiments identify CuL¹ as the source of the catalytic activity, but do not exclude the possibility that it may be the precursor to an adsorbed catalyst.

To probe for adsorption of the CuL¹ on the electrode surface prior to catalysis, a “soak test” was performed using the methods of Dempsey and co-workers (Lee at al., Inorg. Chem. 2017, Vol. 56, pp. 1988-1998—DOI: 10.1021/acs.inorgchem.6b02586). The working electrode was immersed overnight in a 0.1 M Bu₄NPF₆ ACN solution containing 0.6 mM CuL¹ and 0.292 M acetic acid. It was then removed, washed with DI water, and placed into a fresh solution of 0.1 M Bu₄NPF₆ ACN, with no added acid or catalyst. The resulting CV displayed no redox events indicating no detectable adsorption of CuL¹ derived species under these conditions.

A second series of controls were performed to evaluate if a catalytically active adsorbed species forms during CV catalysis. These post-CV “dip-tests” were conducted after 10 cycles and 50 cycles (See FIGS. 62 and 63). During the 50 cycles from −0.5 to −2.3 V vs. Fc⁺/Fc on 0.6 mM CuL¹ solutions under acid saturated conditions the current reaches a maximum value of ˜1.5 mA. The working electrode was removed, washed with DI water, and immersed into a fresh solution of 0.1 M Bu₄NPF₆ ACN or DMF. The resulting CVs showed no significant Faradaic current in the window from −0.5 to −2.3 V. Upon addition of 0.292 M acetic acid, a catalytic current of 1 mA was observed at −1.7 V. After we extended the scan window to include 0.4 V, the catalytic current at −1.7 V is absent. This confirms that at least some of the HER catalysis results from adsorbed CuL¹ species.

To probe if all of the catalytic activity results from adsorbed catalysts, we repeated the post-CV “dip-test” after 10 cycles from −0.5 to −2.5 V vs. Fc⁺/Fc on 0.6 mM CuL¹ solutions under acid saturated conditions, again reaching maximum current values of ˜1.5 mA. The working electrode was removed, washed with DI water, and immersed into a fresh solution of 0.1 M Bu₄NPF₆ DMF. As before, the resulting CVs showed no observable Faradaic current in the window from −0.5 to −2.5 V. CVs following addition of 0.292 M acetic acid that also showed only 400 μA of current −1.7 V. While these results indicate that surface adsorbed CuL¹ is responsible for some of the catalytic current after as few as 10 cycles, they show that the majority of HER activity under homogeneous conditions is due to dissolved CuL¹ complex.

In addition, a post-electrolysis “dip-test” was performed following CPE studies of CuL¹ catalyzed HER. Under these conditions, a substantial amount of surface adsorbed CuL¹ derived complex is expected. After both CPEs in DMF and ACN, the working electrode was removed, washed with DI water, and immersed into a fresh 0.1 M Bu₄NPF₆ DMF/ACN solution. The CV was collected. In contrast to the post-CV “dip-tests”, the electrode displays three reduction events at −0.9 V, −1.4 V and −2.1 V vs Fc⁺/Fc. The first two events are near the observed Cu(II/I) reductions potentials of [CuL¹H]⁺ and CuL¹, respectively. The most cathodic event is near the reduction potential of H₂L¹. Upon addition of 0.292 M acetic acid to the solution, current increases and catalysis is observed (FIG. 30).

Analysis of the films following electrolysis in DMF and ACN by x-ray photoelectron spectroscopy revealed atomic percentages consistent with CuL¹, FIG. 31. The results are inconsistent with the formation of nanoparticles consisting of metallic Cu, copper oxides, or copper sulfides. The data from the two solvents are indistinguishable. High resolution XPS (see above and FIG. 61) confirms the presence of Cu ions and is inconsistent with metallic Cu.

Protonated Derivatives of CuL¹

A series of protonated derivatives of CuL¹ were evaluated as potential catalytically relevant intermediates. These include the mono- and di-protonated Cu(II) complexes [CuLH] and [CuLH₂]²⁺ and the Cu(I) analogue CuLH.

To confirm that [CuUH]⁺ is present in solution under catalytic conditions, acid titrations were monitored by UV-visible spectroscopy. The spectrum of 0.6 mM CuL¹ in deoxygenated DMF shows absorbance bands at 310, 375, 475 and 520 nm. The solution was titrated with acetic acid, increasing in concentration from 0.022 M to 0.382 M (FIG. 32). The absorbance bands of CuL¹ decrease in intensity, concurrent with increases at 405 and 460 nm consistent with the formation of [CuL¹H]⁺. The proposed protonation site is the hydrizino N, in line with previous reports and density functional theory computations (vide infra).

To characterize the monoprotonated Cu(I) species, CuL¹H, CuL¹ was first reduced and then protonated. In an Ar filled glove box, CuL¹ was dissolved in DMSO-d₆. Upon addition of one equivalent of cobaltocence, the solution changed color from red to light purple. The solution was stirred for 15 minutes to obtain the reduced anionic copper complex, [CuL¹]⁻. An aliquot was added to an NMR tube for analysis. The ¹H NMR spectrum of [CuL¹]⁻ displays the expected peaks. Chemical shifts at 2.73, 2.89, and 7.95 ppm are assigned to the methyl backbone CH₃, NH(CH₃), and NH(CH₃), respectively, and integrate with a ratio of 3:3:1 (FIG. 55). Addition of one equivalent of HBF₄ to the [CuL¹]⁻ solution results in a color change from light purple to orange. An aliquot was taken and analyzed by ¹H NMR. Each of the peaks observed in [CuL¹]⁻ are still present, along with a new peak at 8.27 ppm, which integrates to 1 (FIG. 56). The new peak is assigned to the protonation of the hydrazino nitrogen. The chemical shift is inconsistent with metal-centered protonation, which would result in a copper hydride with a negative chemical shift.

Our CH₃COOH titration studies of diprotonated Cu(II) complex [CuL¹H₂]²⁺ show no evidence of a second protonation event under catalytic conditions. Although [CuL¹H₂]²⁺ is not catalytically relevant, x-ray quality crystals of [CuL¹H₂]²⁺ were obtained from perchloric acid solution. The ORTEP representation shows protonation of both hydrizino nitrogens, axial coordination of one perchlorate, and axial association of the second perchlorate, FIG. 33. A complete description of the crystallographic details is provided above.

Proposed Homogeneous HER Mechanism

A proposed CECE mechanism for homogenous HER catalyzed by CuL¹ is shown in FIG. 34. Step 1 is an initial chemical (C) event involving protonation of the hydrazino nitrogen on CuL¹ to yield [CuL¹H]⁺. This is followed by an electrochemical step (E) assigned as a metal-centered reduction giving the neutral Cu(I) species, CuL¹H. Step 3 is a chemical step involving protonation on the other hydrazino nitrogen, affording the Cu(I) cation, [CuL¹H₂]⁺. Step 4 is the final electrochemical step, a proposed ligand-centered reduction to give the neutral species, CuL¹H₂. The CuL¹H₂ complex can be regarded as Cu(I) coordinated by a nitrogen-centered radical. Step 5 shows double bond rearrangement, resulting in an anionic coordinated nitrogen. This anionic nitrogen induces an internal proton transfer, a tautomerism that has been observed in many thiosemicarbazone complexes, leading to formation of the H₂ evolving complex. Finally, in step 6, hydrogen is evolved through hydrogen atom or proton/hydride coupling at the adjacent N—H bonds.

The proposed mechanism is consistent with the experimentally determined rate law and the KIE study. Both protons are added to a single CuL¹ complex prior to the rate determining step (r.d.s.) consistent with the first-order catalyst and second-order proton dependence on current. The proton inventory study suggests that a single proton is involved in the r.d.s. This favors tautomerization, step 5, over hydrogen evolution, step 6, as the rate limiting event. The solvent dependency of the TOF further supports this interpretation. The TOF in DMF is approximately one-half that in ACN. This is attributed to H-bonding interactions between DMF and ligand N—H groups. These stabilizing interactions retard the rate limiting tautomerization step in DMF relative to ACN. To assess the viability of this mechanism, density functional theory computations on all proposed species were performed.

Density Functional Theory Investigations

All proposed complexes in FIG. 34 were assessed using density functional theory (DFT) using the B3LYP hybrid functional and the 6-311g(d,p) basis set. First, we evaluated the protonation event associated with step 1. Calculations on the singly protonated intermediate, [CuL¹H]⁺, support our assignment of the hydrazino nitrogen as the site of protonation. For [CuL¹H]⁺ (S=1/2), the Cu, S, N3 (hydrazino), N4 (coordinated), and N6 (pendant amine) were evaluated as possible protonation sites. In each structure, the geometry and frequencies were optimized and the energies minimized. The hydrizino protonated geometry is energetically preferred, lying 10 kcal/mole lower than the metal-hydride (FIG. 35A). Protonation at S is less favored by 12 kcal/mole and attempts to optimize structures with protonation on N4 and N6 resulted in migration of the hydrogen onto the hydrazino nitrogen.

Next, the one-electron reduced protonated species, CuL¹H (S=0) (FIG. 35B), was examined to determine if reduction, step 2, impacts the location of the proton. Computed free energies for structures with protonation at Cu, S, and each N, clearly indicate that protonation on the hydrazino N is still favored. Protonation at S is disfavored by 11 kcal/mole, while protonation at the pendant amine is less favored by 18 kcal/mole. Attempts to optimize CuL¹H with protonation at the coordinated nitrogen again resulted in migration of hydrogen onto the hydrazino nitrogen. We also considered CuL¹H with protonation on the hydrazino nitrogen in its triplet electronic configuration (S=1). However, it is 5.77 kcal/mole higher in energy (see FIG. 59), confirming the nature of the first reduction as metal based.

Step 3 of the proposed mechanism involves addition of a second proton to CuL¹H. Energy minimizations of the doubly protonated, singly reduced intermediate, [CuL¹H₂]⁺ in both the singlet (S=0) and triplet (S=1) electronic states were performed. For each spin state, the copper, the opposing hydrazino nitrogen (N1), sulfur (S2) , the pendant amine (N6) and the coordinated nitrogen (N4) were all considered as the second protonation site. In both electronic states, protonation on the opposing hydrazino nitrogen (N1) is favored over all other protonation sites by at least 8 kcal/mole. Comparison of the singlet and triplet state the energies for protonation on N4 indicate the singlet is more stable by 6.09 kcal/mole (FIG. 36).

The addition of the second electron, step 4, leads to formation of the hydrogen evolving complex CuL¹H₂ through proposed rearrangement, step 5. The geometry and frequencies of CuL¹H₂ were optimized with one proton located on N3 while considering multiple sites for the second proton including the copper, the opposing hydrazino nitrogen (N1), sulfur (S2) the pendant amine (N6) and the coordinated nitrogen (N4). Energy minimizations indicate that placement of the second proton on N1 or N4 nitrogen are most favored, by at least 17 kcal/mole, compared to all other sites (FIG. 37A). Notably, energies for protonation at N1 and N4 differ by only 0.39 kcal/mole, indicating that the tautomerization associated with step 5 in the proposed mechanism is viable. Further, examination of the spin-density (SD) shows that the second reduction is primarily ligand based, with 34% on N2, 38% on C4, 14% on N1, 8% on N5, and only 3% on Cu (FIG. 37B).

Discussion

The complexes cited herein present intriguing reactivity as the ligand can participate in electron transfer events, either with or without a transition metal—rendering it “non-innocent” in some instances. Additionally, the ligands can be protonated generating various tautomeric forms—rendering it not only non-innocent, but also “promiscuous” in some instances. The combination of these two factors can be dependent on the identity of the metal ion, and, in some instances, can result in three types of HER reactivity: a) ligand-assisted metal reactivity with Ni, b) ligand-centered reactivity with Zn, and c) metal-assisted ligand reactivity with Cu.

In some examples, the impact of the metal is manifested in the initial protonation and initial reduction sites. For both CuL¹ and ZnL¹, a hydrazino N is protonated prior to the initial reduction. The initial site of reduction can be rationalized based on the relative energies of the metal d-orbitals and an unoccupied ligand centered orbital, FIG. 38. The metal d-orbital energies decrease from Ni²⁺ to Cu²⁺ to Zn²⁺ with increasing effective nuclear charge. For Ni²⁺, a vacant ligand centered orbital lies below the vacant metal d_(z2) orbital yielding a ligand-centered radical upon reduction. For Cu²⁺, the stabilized d-orbital manifold falls below the ligand-centered orbital resulting in metal-centered reduction. For Zn²⁺, the d-orbitals are filled required ligand-centered reduction. Additionally, the site of reduction and the localization of spin-density can impact the site of protonation. For Cu, the proton remains on the hydrizino nitrogen after metal-centered reduction. In contrast, for Ni the ligand-centered reduction favors double bond rearrangement similar to step 5 in FIG. 34. As a result, the tautomers with protonation of the hydrizino N and anionic coordinated N have similar energies. For Zn, the tautomer with the proton remaining on the hydrizino N remains favored after ligand-centered reduction.

In some embodiments, the mechanism for H₂ evolution is a consequence of the initial protonation and reduction sites. For Ni, some has posited that the second reduction is metal-based, generating a nucleophilic d⁹ Ni(I) center. Further protonation is proposed to occur at the metal generating a Ni^(III)-hydride as the catalytically active species for H₂ evolution. In this context, NiL² can demonstrate ligand-assisted metal reactivity, in which the ligand serves as an auxiliary redox site to facilitate two-electron chemistry at the metal. This can be a common role for redox active ligands in transition metal catalysis. In contrast, the HER chemistry of ZnL¹ can be ligand-centered. Its HER mechanism can localize all chemical and electrochemical steps on the ligand, with the metal providing structural support. As in the case of NiL², initial protonation and reduction is ligand-centered, however, the d¹⁰ Zn(II), in some instances, is incapable of undergoing a second reduction and HER proceeds via a bimolecular process.

The HER chemistry of CuL¹ displays a less common type of mechanism with redox non-innocent ligands, which we defined as metal-assisted ligand-centered reactivity. The initial metal-centered reduction leads to a d¹⁰ Cu(I), which is isoelectronic with ZnL¹. As such, it can accommodate subsequent ligand-based protonation and reduction events with evolution of H₂ from the ligand-center. In this context, the metal serves as the auxiliary redox site, which can facilitate two-electron chemistry at the ligand.

CuL¹ demonstrates a metal-assisted ligand-centered mechanism, in some instances. The redox non-innocence and protonation promiscuity of the ligand provides for a variety of HER mechanisms, some of which can be dependent on the metal ion. The CuL¹ system exhibits the highest reported TOF of any ligand-centered homogeneous HER catalysts to date. However, its high activity can sometimes require large overpotential. Nonetheless, the metal-assisted ligand reactivity of CuL¹ provides a new template for future HER electrocatalysts that, in some instances, function without the participation of a metal-hydride. The current study demonstrates that non-innocent ligands can work in conjunction with a redox-active metal to promote ligand-centered reactivity. This represents a new approach to the development of electrocatalysts for HER and, possibly, the activation of other small molecules.

Example Set C Example Syntheses and Electrochemical Investigations of Alkoxy Derivatives

Alkoxy derivatives were prepared as shown in the scheme below.

The hydrazinecarbothioc acid O-alkyl esters were prepared and condensed with diones. This was followed by metalation (shown here as Cu acetate). Characterization of their Cu(II) complexes reveals only small structural and spectroscopic changes relative to their bis-thiosemicarbazone counterparts. However, electro-chemical investigations reveal anodic shifts of ˜340 mV in the Cu^(II/I) reduction. The results demonstrate the ability to modulate the potential by variation of the thiosemicarbazone N-termini without imparting large structural changes. The synthetic strategies highlighted in the scheme provide examples of the design of new molecular catalysts, the synthesis of catalysts with extended structures, and the covalent attachment of catalysts to electrode surfaces.

Example Set D Example Syntheses

A series of compounds can be synthesized containing transition metal-(Cu), non-transition metal-(Zn), and metal-free derivatives of symmetric (R₁═R₂) derivatives (H₂L¹⁻⁴) and the corresponding asymmetric derivatives (H₂L⁵⁻⁸) in which R₂ is an ethoxy group, as in the scheme below.

The bis-thiosemicarbazone ligands H₂L¹⁻⁴ and their Cu and Zn complexes have been prepared. The asymmetric ligand H₂L⁵, was prepared by condensation of la with the N-amino-O-ethylthiocarbamate (prepared from ethanol, CS₂, and NH₂NH₂.H₂O) in 50-70% yield (see above). Addition of Cu(OAc)₂ in methanol yields CuL⁵ as a dark brown precipitates upon reflux. Spectroscopic characterization reveals a similar ligand environment as CuL¹ and related derivatives. The x-ray structure of CuL⁵, confirms the asymmetric N₂S₂ environment.

Example Set E Examples of Asymmetric Formulas

A series of asymmetric ligands has been synthesized, according to the scheme below.

Ligands H₂L^(10a-c) have been prepared by condensation of 1a-c with the N-amino-O-ethylthiocarbamate (prepared from ethanol, CS₂, and NH₂NH₂.H₂O) in 50-70% yield. Addition of Cu(OAc)₂ in methanol yields CuL^(10a-c) as a dark brown precipitates upon reflux. Spectroscopic characterization reveals a similar ligand environment as CuL¹ and related derivatives. The x-ray structure of CuL^(10a), confirms the asymmetric N₂S₂ environment. H₂L^(8a) and CuL^(8a) were also prepared.

The CVs of CuL^(10a-c) in acetonitrile (0.1 M TBAHFP) show a quasireversible reduction and oxidation. For CuL^(10a), the Cu^(II/I) reduction occurs at −1010 mV with the formal Cu^(III/II) couple observed at +448 mV. These values lie between the respective potentials of the symmetric derivatives CuL¹ and CuL⁵. Substitution of the backbone methyl groups with ethyl results in a small, but measurable cathodic shift to −1030 mV for the Cu^(II/I). Incorporation of phenyl substituent in the backbone, CuL^(10c)c, yields a larger shift in the Cu^(II/I) potential to −850 mV. Notably, the Cu^(II/I) reduction of CuL^(10c) is more accessible than in the symmetric CuL⁵. This confirms modification of backbone R groups as a viable strategy to tune potential. Also, the CH₂CF₃ group of CuL^(8a) shifts the Cu^(II/I) potential to +70 mV with relative to CuL¹ in DMF.⁴⁰

Initial CVs confirm the HER activity of CuL^(10b,c) in acetonitrile. Addition of acetic acid to CuL^(10c) increases catalytic current at −1.5 V, FIG. 64. The current is first-order in [acid] until it reaches acid independence at [acid]>0.093 M. The maximum catalytic currents stabilizes at 1.80 mA associated with a maximum i_(cat)/i_(p) value of 43.8 (v=1.0 V/s), affording a TOF of 15,600 s⁻¹ assuming the mechanism is bimolecular, as observed for CuL¹. An overpotential of 1.5 V was determined by open circuit potential methods. CuL^(10b) has similar HER electrocatalytic activity.

Example Set F Glassy Carbon Electrode (GCE) Preparation and Results

H₂L¹ and its Cu and Zn complexes (See Example Set B) are insoluble in water making them ideal candidates for preparing modified GCEs. GC-H₂L¹, GC-ZnL¹, and GC-CuL¹ were prepared by dropcast of from 20 nmol to 200 nmol of the appropriate catalyst on 5 mm GCE disks and curing of the resulting films. Electrodes were thoroughly dried and stored in air prior to use. The Electrochemical Impedance Spectroscopic (EIS) data for GC-CuL¹ is consistent with a contact resistance of 14 Ω. The charge transfer resistance, which has an impact on overpotential, of GC-CuL¹ is 1760 Ω. This relatively low charge transfer resistance could be attributed to the ease of electron transfer within the planar, conjugated framework of CuL¹. Results for ZnL¹ are similar. Initial CV studies confirm HER activity of the surface confined catalysts. From the polarization curves, the overpotential required to achieve a current density of 10 mA/cm² is 699 mV for GC-CuL¹, FIG. 65. GC-ZnL¹ and GC-H₂L¹ have higher overpotential of 940 mV and 1200 mV, respectively.

Example Set G Carbon Paste Electrodes (CPE)—Fabrication and Surface Analysis

CPEs modified with H₂L¹, CuL¹ and ZnL¹ (from Example Set B) were prepared by mixing graphite fine powder with and 0.5 wt % of the appropriate catalyst in dichloromethane and sonicating for 15 min. The mixture was dried under an infrared lamp for 30 min. Paraffin oil (20 wt %) was added and the resulting slurry was carefully mixed to form the carbon paste. The paste was pressed into a plastic tube and a Cu wire was inserted to establish electrical contact. The electrode surface was polished with weighing paper and washed with deionized water. Finally, 2 μL of a Nafion (5% in isopropanol) was added to the polished surface and the chemically modified CPE was dried under an infrared heating lamp for 15 min. CPEs were stored in air.

The CPE-CuL¹ surface was imaged using SEM and the impedance measured by EIS. The SEM images show clear and homogeneous graphite layers with uniformly distributed pores, FIG. 66a . The surface porosity should allow for sufficient ion diffusion from the bulk to the catalysts embedded in the electrode film. Resistance between the CPE-CuL¹ and solution was measured by EIS. Data collected over a frequency range of 10⁻² to 10⁵ Hz at an applied overpotential of 1.3 V were used to construct a Nyquist plot, FIG. 66b . Fitting of the data yields a resistance between the electrode surface and electrolyte of 12.3 Ω, a charge transfer resistance within the electrode of 508 Ω, a capacitive component indicative of space charges or electrical dipoles in the sample. Overall, resistance for the CPEs is significantly smaller than the modified GCEs in Example Set F.

Polarization curves were recorded for the CPE-CuL¹ in 0.5 M H₂SO₄ solution under ambient conditions, FIG. 66c . The overpotential required to obtain a current density of 60 mA/cm² is 0.65 V. At lower current densities, the overpotential is below the 0.6 V benchmark for relevant HER catalysts, although the value at the recommended current density of 10 mA/cm² complicated by a series of events attributed to formation of a catalytically active dinuclear species, as seen in homogenous solution. For CPE-ZnL¹ and CPE-H₂L¹, the overpotential required for 60 mA/cm² is 0.94 and 1.04 V, respectively.

Initial assessment of the electrode stability of CPE-CuL¹ was investigated by repetitive scanning in 0.5 M H₂SO₄ solution for 500 cycles from 0.0 to −1.7 V at a scan rate of 50 mV/s. The overpotential required for a current density of 100 mA/cm² increased by only 50 mV. The long term stability of CPE-CuL¹ was benchmarked by chronopotentiometry at a fixed catalytic current density of 100 mV/cm² for 24 hours. The overpotential increased by 97 mV over the first 8 hours and remained constant thereafter indicating good stability over long time electrolysis.

Example Set H Example Syntheses and Electrochemical Investigations of Amine Derivatives

Methods

Physical Methods. Elemental analyses were performed by Midwest Microlab, (Indianapolis, Ind., USA). ¹H and ¹³C NMR data were collected on a Varian Inova 500 MHz and Varian 400 MHz NMR Spectrometers in commercial deuterated solvents (Aldrich or Cambridge Isotopes). High-resolution electrospray ionization mass spectrometry in the negative ion mode (-ESI) was performed by the Laboratory for Biological Mass Spectrometry at Texas A&M University. Infrared spectra were recorded on Thermo Nicolet Avatar 360 spectrometer with ATR attachment (4 cm⁻¹ resolution). Electronic absorption spectra were recorded with an Agilent 8453 diode array spectrometer with a 1 cm path length quartz cell. All electrochemical measurements were performed using a Gamry Interface potentiostat/galvanostat with a three-electrode cell (glassy carbon working electrode, platinum wire counter electrode, and Ag/Ag ion reference electrode). Reported potentials are scaled vs. a ferrocenium/ferrocene (Fc⁺/Fc⁰) standard (0.00 V), which was determined using ferrocene as an internal standard.

Materials and Methods. All reagents were obtained from commercially available sources and used as received unless otherwise noted. Commercial solvents were additionally dried and purified using an MBraun solvent purification system unless otherwise noted. The compounds in this study are air and moisture stable as solids and were handled on the benchtop with no additional required protection from the atmosphere.

Synthesis. All reactions were performed open to air and under ambient conditions unless otherwise indicated. The N,N′-(dimethylethylenediaminothiosemicarbazanato)-4-(methylthiosemi-carbazanato)butane-2,3-diimine=ATSM/DM and N,N′-bis(dimethylethylenediaminothiosemi-carbazanato)butane-2,3-diimine=ATSDM ligands were synthesized according to modified literature methods (XIE et al., “Exploiting Copper Redox for ¹⁹F Magnetic Resonance-Based Detection of Cellular Hypoxia” J. Am. Chem. Soc. (2016) Vol. 138, pp. 2937-2940.)

Some syntheses were performed according to Scheme H1.

ATSM/DMEDA: To a suspension of ATSM/DM (0.5 g, 1.8 mmol) in MeCN (50 mL) was added N,N-dimethylethylene diamine (0.440 mL, 4.0 mmol). Formed yellow solution was refluxed overnight. A white precipitate, which formed upon cooling to room temperature, was filtered and dried under vacuum. Yield=0.445 g (78%). ¹H NMR (500 MHz, d₆-DMSO) of ATSM/DMEDA: 10.2 (bs, 2H, —NH—N═C—), 8.33 (q, 2H J_(HH)=4 Hz, 7.5 Hz, —NH—C═S), 3.58 (q, 2H, J_(HH)=7.5 Hz, 8.3 Hz, —CH₂NH—), 3.00 (t, J_(HH)=4 Hz, 3H, NH—CH₃, 2.43 (t, 2H, J_(HH)=8.3 Hz, —CH₂N(CH₃)₃, 2.18 (s, 3H, —CH₃), 2.16 (s, 6H, —N(CH₃)₂), 2.14 (s, 3H, —CH₃). ¹³C NMR (100 MHz, d₆-DMSO): δ/ppm 178.8 (C═S), 178.0 (C═S), 148.4 (C═N), 148.0 (C═N), 57.5 (—CH₂NH—), 45.5 (N(CH₃)₂), 41.9 (—CH₂N(CH₃)₂, 31.6 (CH₃NH—), 12.2 (—CH₃) 11.8 (—CH₃). FT-IR, cm⁻¹: 3364 (w, N—H), 3208 (br, m, N—H), 2954 (w), 2817 (w), 2785 (w), 1486 (vs, C═N), 1291 (s, thioamide), 947 (m), 660 (m).

ATSDMEDA: To a suspension of ATSDM (0.5 g, 1.73 mmol) in MeCN (50 mL) was added N,N-dimethylethylene diamine (0.630 mL, 5.73 mmol). Formed orange-yellow solution was refluxed overnight. An off-white solid which formed upon cooling to room temperature was filtered and air dried. Yield=0.473 g (73%). ¹H NMR (500 MHz, d₆-DMSO) of ATSDEMDA: δ/ppm 10.33 (bs, 1H, —NH—N═C—), 8.35 (t, 1H, J_(HH)=5.4 Hz, NH—C═S), 3.61 (q, J_(HH)=5.8 Hz, 6.4 Hz, 2H, —CH₂NH—), 2.45 (t, J_(HH)=2.5 Hz, 6.4 Hz, 2H, —CH₂N(CH₃)₂, 2.18 (bs, 6H, —N(CH₃)₂, 2.16 (s, 3H, —CH₃). ¹³C NMR (125 MHz, d₆-DMSO): δ/ppm 178.1 (C═S), 146.1 (C═N), 57.6(—CH₂NH—), 45.5 (—N(CH₃)₂), 41.9 (—CH₂N(CH₃)₂, 11.9 (CH₃). FT-IR, cm⁻¹: 3337 (w, N—H), 3140 (br, m, N—H), 2974 (w), 2821 (w), 2767 (w), 1479 (vs, C═N), 1246 (s, thioamide), 806 (m), 606 (m).

NiATSM/DMEDA (21): To a solution of ATSM/DMEDA (0.4 g, 1.25 mmol) in MeOH (50 mL) was added Ni(OAc)₂. 4 H₂O (0.313 g, 1.25 mmol). Formed dark green suspension was heated with stirring for 4 h. The formed dark green solid was filtered and air dried. Yield=0.383 g (82%). X-ray quality single crystals were obtained via layering technique by slow diffusion of Et₂O into a MeCN solution of 21. ¹H NMR (400 MHz, d₆-DMSO) of NiATSM/DMEDA: δ/ppm 7.66 (bs, 1H, NH—C—S), 7.60 (bs, 1H, NH—C═S), 3.28 (q, J_(HH)=6.0 Hz, 2H, —CH₂NH—), 2.78 (d, J_(HH)=4.2 Hz, 3H, —CH₃), 2.33 ppm (t, J_(HH)=6.7 Hz, 2H, CH₂N(CH₃)₂,) 2.13 (bs, 6H, —N(CH₃)₂, 1.93-1.92 (bs, 6H, 2 x-CH₃). FT-IR, cm⁻¹: 3274 (br, m, N—H), 2945 (br, w), 1476 (vs, C═N), 1219 (s, thioamide), 942 (w), 771 (w). UV-vis (CH₃CN/MeOH, 1:1): λ_(max), nm (ε, M⁻¹ cm⁻¹)=257 (27,000), 393 (13,000). Anal. Calc. for C₁₁H₂₁N₇NiS₂: C, 35.34; H, 5.66; N, 26.24. Found: C, 35.34; H, 5.65; N, 25.99. (−)ESI-MS, m/z calc. for [M−H]⁻, [C₁₁H₂₁N₇NiS₂]−H; 372.07. Found: 371.96.

NiATSM/TMAEDA (22): To a suspension of NiATSM/DMEDA (0.200 g, 0.534 mmol) in MeCN (25 mL) was added methyl iodide (33.3 μL, 0.534 mmol). The resulting dark green suspension was stirred overnight. Formed dark green solid was filtered and air dried. Yield=0.212 g (77%). X-ray quality single crystals were grown using vapor diffusion technique in an H-shaped tube by slow diffusion of CH₂Cl₂ into a MeCN/MeOH (1:1) solution of 22. ¹H NMR (400 MHz, d₆-DMSO) of NiATSM/TMAEDA: δ/ppm 7.96 (bs, 1H, NH—C═S), 7.77 (bs, 1H, NH—C═S), 3.61 (bs, 2H, —CH₂NH—), 3.42 ppm (t, J_(HH)=5.9 Hz, 2H, CH₂ ⁺N(CH₃)₃), 3.09 (bs, 9H, —⁺N(CH₃)₃), 2.79 (d, J_(HH)=3.5 Hz, 3H, —CH₃), 1.96 (bs, 6H, 2 x-CH₃). FT-IR, cm⁻¹: 3218 (br, s, N—H), 2938 (w), 1488 (br, vs, C═N), 1224 (s, thioamide), 964 (m), 922 (m). UV-vis (CH₃CN/MeOH, 1:1): λ_(max), nm (ε, M⁻¹ cm⁻¹)=223 (20,000), 256 (24,000), 398 (12,000). Anal. Calc. for C₁₂H₂₄IN₇NiS₂: C, 27.93; H, 4.69; N, 19.00. Found: C, 27.96; H, 4.61; N, 18.96. (−) ESI-MS, m/z calc. for [M−H]⁻, [C₁₂H₂₄IN₇NiS₂]−H; 513.99, Found: 513.99.

NiATSDMEDA (23): To a solution of ATSDMEDA (0.400 g, 1.06 mmol) in MeOH (50 mL) was added Ni(acac)₂. 2H₂O (0.307 g, 1.06 mmol). Formed dark green solution was heated with stirring for 4 h. A dark green precipitate, formed upon concentration of the solution, was filtered and air dried. Yield=0.337 g (74%). X-ray quality single crystals were obtained using layering technique by slow diffusion of Et₂O into a MeCN/MeOH (1:1) solution of 23. ¹H NMR (500 MHz, d₆-DMSO) of NiATSDEMDA: δ/ppm 7.61 (bs, 1H, —NH—C═S), 3.27 (q, J_(HH)=6.2 Hz, 2H, —CH₂NH—), 2.34 (t, J_(HH)=6.8 Hz, 2H, —CH₂N(CH₃)₂, 2.13 (bs, 6H, —N(CH₃)₂, 1.92 (bs, 3H, —CH₃). FT-IR, cm⁻¹: 3361 (br, m, N—H), 3189 (br, m, N—H), 2937 (br, m), 2767 (br, s), 1404 (vs, C═N), 1230 (s, thioamide), 934 (br, m), 769 (m). UV-vis (CH₃CN/MeOH, 1:1): λ_(max), nm (ε, M⁻¹ cm⁻¹)=256 (20,000), 398 (11,000). Anal. Calc. for C₁₄H₃₀N₈NiS₂: C, 38.99; H, 6.54; N, 25.98. Found: C, 38.76; H, 6.40; N, 25.61. (+)ESI-MS spectrum of NiATSDMEDA (23) calc. for [C₁₄H₂₈N₈NiS₂]+H: 431.12. Found: 431.1298.

NiATSTMAEDA (24): To a suspension of NiATSM/DMEDA (0.200 g, 0.464 mmol) in acetonitrile (25 mL) was added methyl iodide (57.8 μL, 0.928 mmol). The resulting dark green suspension was stirred overnight. Formed dark green solid was filtered and air dried. Yield=0.205 g (62%). X-ray quality single crystals were grown using vapor diffusion technique in an H-shaped tube by slow diffusion of Et₂O into a MeCN/MeOH (7:3) solution of 24. ¹H NMR (500 MHz, d₆-DMSO) of NiATSTMAEDA: δ/ppm 8.07 (bs, 1H, —NH—C═S), 3.61 (bs, 2H, —CH₂NH—), 3.41 (t, J_(HH)=6.6 Hz, 2H, —CH₂ ⁺N(CH₃)₃, 3.08 (bs, 9H, —⁺N(CH₃)₃, 1.97 (bs, 3H, —CH₃). FT-IR, cm⁻¹: 3280 (br, m, N—H), 1410 (vs, C═N), 1227 (s, thioamide), 914 (br, m). UV-vis (CH₃CN/MeOH, 1:1): λ_(max), nm (c, M⁻¹ cm⁻¹)=226 (33,000), 256 (28,000), 392 (13,000). Anal. Calc. for C₁₆H₃₄I₂N₈NiS₂. H₂O: C, 26.21; H, 4.95; N, 15.28. Found: C, 25.84; H, 4.71; N, 15.04. (−)ESI-MS, m/z calc. for [M−H]⁻, [C₁₆H₃₄I₂N₈NiS₂]−H; 712.98. Found: 712.97.

Results and Discussion—Synthesis

Two N₂S₂ diacetyl-2,3-bis-(N4-alkyl-3-thiosemicarbazone) ligands (H₂L¹ and H₂L²) were prepared via transamination of diacetyl-2,3-N4-methyl-3-thiosemicarbazone-N4-dimethyl-3-thiosemicarbazone and diacetyl-2,3-bis-(N4-dimethyl-3-thiosemicarbazone) with N,N-dimethylethylendiamine (Scheme H1). Compound 21 was obtained by synthesis of H₂L¹ with nickel(II) acetate tetrahydrate. The same synthetic approach attempted for compound 23, resulted in a protonated version. To overcome this problem, nickel(II) acetylacetonate was used instead as a nickel source, which allowed us to isolated the neutral compound 23. Compounds 21 and 23 were further alkylated with methyl iodide to obtain the charged compounds 22 and 24(See Scheme H1).

The UV-Vis spectra of 21-24 recorded in a mixture of acetonitrile/methanol solution display a ligand to metal charge transfer band near 400 nm. All four compounds display more intense ligand to ligand charge transfer bands near 256 MTh Two additional bands are observed in the alkylated species of 22 (223 nm) and 24 (226 nm), which can be attributed to the presence of the iodide counter anions. The infrared spectra of ligands H₂L¹ and H₂L² display an N—H stretch between 3337 and 3364 cm⁻¹ that is lost upon metal complexation.

Results and Discussion—Crystallographic Studies

X-ray quality single crystals of 21-24 were obtained as orange plates using layering or vapor diffusion techniques. All complexes crystallize as discrete, square planar Ni(II) complexes with no solvent molecules in the crystal lattice. Selected bond distances and angles are summarized in Table H1. Crystal data and structure refinement details are listed in Table H2.

TABLE H1 Selected Bond Distances (Å) and Angles (deg) for 21, 22, and 24. Bond 21 22 24 distance (cag265ltb) (cag267lte) (cag271lta) Nil—N1 1.854(2) 1.860(3) 1.864(2) Nil—N2 1.861(2) 1.846(3) 1.869(2) Nil—S1 2.1540(8) 2.1642(11) 2.1710(7) Nil—S2 2.1493(8) 2.1562(12) 2.1715(7) S1—C5 1.766(3) 1.764(4) 1.769(3) S2—C10^(a) 1.780(3) — — S2—C11^(a) — 1.766(4) 1.778(2) N1—C1 1.304(4) 1.301(5) 1.314(3) N1—N3 1.375(3) 1.374(4) 1.388(3) N2—C2 1.307(4) 1.317(5) 1.312(3) N2—N6 1.377(3) 1.382(4) 1.387(3) N3—C5 1.321(4) 1.307(5) 1.324(3) N4—C5 1.335(4) 1.349(5) 1.362(3) N4—C6 1.461(4) 1.443(6) 1.461(3) N6—C10^(a) 1.317(4) — — N6—C11^(a) — 1.324(5) 1.316(3) N7—C10^(a) 1.329(4) — — N7—C11^(a) — 1.332(5) 1.360(3) N7—C11^(a) 1.453(4) — — N7—C12^(a) — 1.461(5) 1.465(3) C1—C2 1.471(4) 1.481(6) 1.483(3) Bond angle N2—Nil-N1 83.87(11) 83.45(15) 83.63(9) N2—Nil—S2 87.37(8) 87.63(11) 87.24(7) N1—Ni1—S2 170.94(8) 171.07(10) 170.82(7) N2—Nil—S1 171.18(8) 170.68(11) 170.58(7) N1—Nil—S1 87.47(8) 87.56(10) 87.56(6) S2—Nil—S1 101.34(3) 101.37(4) 101.61(3) ^([a])The C10 in 21 is the same carbon atom as C11 in 22 and 24. The C11 in 21 is the same carbon atom as C12 in 22 and 24.

TABLE H2 Crystal Data and Structure Refinement for 21-24. Identification code 21 (cag265ltb) 22 (cag267lte) 23 (cag273lta) 24 (cag271lta) Empirical formula C₁₁H₂₁N₇NiS₂ C₁₂H₂₄IN₇NiS₂ C14 H24.50 N8 N1 S2 C₁₆H₃₄I₂N₈NiS₂ Formula weight 374.18 516.09 427.75 715.14 Temperature (K) 102(3) 101(1) 102(2) 101.95(10) Wavelength (Å) 0.71073 0.71073 0.71073 0.71073 Crystal system Orthorhombic Monoclinic Triclinic Triclinic Space group Pbca C2/c P1 P1 Unit cell dimensions a (Å) 11.7031(4) 13.9471(16) 7.3123(7) 8.8516(9) b (Å) 15.8057(5) 13.9342(16) 16.3895(14) 9.5977(12) c (Å) 17.6173(4) 20.699(3) 17.6227(16) 16.172(2) α (deg) 90 90.00 70.154(8) 85.494(10) β (deg) 90 91.826(10) 84.378(8) 78.439(10) γ (deg) 90 90.00 89.994(7) 80.176(10) V (Å³) 3258.78(17) 4020.6(8) 1975.8(3) 1324.9(3) Z 8 8 4 2 d_(calcd) (Mg/m³) 1.525 1.705 1.438 1.793 Absorption coefficient (mm⁻¹) 1.451 2.717 1.208 3.239 F(000) 1568 2064 898 704 Crystal color, habit dark orange plate orange plate orange plate green-brown plate Crystal size (mm³) 0.40 × 0.20 × 0.02 0.33 × 0.04 × 0.04 0.38 × 0.08 × 0.04 0.41 × 0.12 × 0.02 θ range for data collection (deg) 3.32 to 27.55 3.47 to 28.12 3.24 to 25.25 3.28 to 30.08 Index ranges −15 ≤ h ≤ 15 −18 ≤ h ≤ 18 −8 ≤ h ≤ 8 −12 ≤ h ≤ 12 −20 ≤ k ≤ 20 −18 ≤ k ≤ 18 −16 ≤ k ≤ 16 −13 ≤ k ≤ 13 −22 ≤ l ≤ 22 −27 ≤ l ≤ 27 −21 ≤ l ≤ 21 −22 ≤ l ≤ 22 Reflections collected 42277 25351 20780 34415 Independent reflections 3764 [R(int) = 0.0485] 4904 [R(int) = 0.049] 7151 [R(int) = 0.1120] 7776 [R(int) = 0.0455] Completeness to theta max (%) 99.8 99.7 99.8 99.8 Absorption correction multi-scan multi-scan multi-scan multi-scan Max. and min transmission 1.00 and 0.69 1.000 and 0.718 1.000 and 0.877 1.000 and 0.593 Refinement method full-matrix full-matrix full-matrix full-matrix least-squares on F² least-squares on F² least-squares on F² least-squares on F² Data/restrains/parameters 3764/0/253 4904/0/233 7151/8/465 7776/0/308 Goodness of fit on F² 1.073 1.074 1.051 1.060 Final R indices [I > 2σ(I)]^(a,b) R1 = 0.0441, wR2 = 0.0957 R1 = 0.0423, wR2 = 0.0970 R1 = 0.0739, wR2 = 0.1215 R1 = 0.0281, wR2 = 0.0508 R indices (all data)^(a,b) R1 = 0.0647, wR2 = 0.1051 R1 = 0.0571, wR2 = 0.1046 R1 = 0.1335, wR2 = 0.1424 R1 = 0.0402, wR2 = 0.0551 Largest diff. peak and 0.639 and −0.352 1.621 and −0.439 1.253 and −0.773 1.692 and −0.903 hole (e · Å⁻³) ^(a)R1 = Σ||Fo| − |F_(c)||/Σ|F_(o)|. ^(b)wR2 = {Σ[w(F_(o) ² − F_(c) ²)²]/Σ[w(F_(o) ²)²]}^(1/2), where w = q/σ²(F_(o) ²) + (qp)² + bp. GOF = S = {Σ[w(F_(o) ² − F_(c) ²)²]/(n − p)}^(1/2), where n is the number of reflections and p is the number of parameters refined.

Single crystals of 21 were obtained from a slow diffusion of Et₂O into a MeCN solution of 21 as orange plates in the orthorhombic space group Pbca. The asymmetric unit of 21 consists of one equivalent of NiATSM/DMEDA, FIG. 67. The Ni of 21 occupies an N₂S₂ square plane provided by the N,N′-(dimethylethylenediaminothiosemi-carbazanato)-4-(methylthiosemicarbazanato)-butane-2,3-diimine ligand with Ni—N1, Ni—N2, Ni—S1, and Ni—S2 bond distances of 1.854(2), 1.861(2), 2.1540(8), and 2.1493(8) A, respectively. The C10-S2, N6-N2, and C2-C1 bond distances of 1.780(3), 1.377(3), and 1.471(4) Å, respectively, are typical of single C—S, N—N, and C—C bonds. The shorter N2-C₂ and N6-C10 bond distances of 1.307(4) and 1.317(4) Å, respectively, reveal C═N character.

The main core framework in compound 21, ignoring the functional groups of the pendant amines, is rigorously planar with the largest deviation from the best fit plane of all 15 non-hydrogen atoms of ±0.1437 Å for S1 and a standard deviation of ±0.0612 Å. The distance between the calculated least squares planes (mean: 15 atoms) of the two stack molecules is 3.437 Å. It is worth to mention that bond distances and bond angles within the N₂S₂ square plane in compound 21 are similar to the parent NiATSM. However, the core framework in compound 21 has a slightly higher deviation from the best fit plane than NiATSM (±0.0698 Å for S1 and a standard deviation of ±0.0255 Å). The observed higher deviation in 21 is due to the bulky dimethylaminoethyl functional group.

X-ray quality single crystals of 22 were isolated from an H-shaped tube by slow diffusion of CH₂Cl₂ into a MeCN/MeOH solution of 22 as orange plates in the monoclinic space group C2/c. The asymmetric unit of 22 consists of one equivalent of NiATSM/TMAEDA, FIG. 68. Like in compound 21, the Ni of 22 occupies an N₂S₂ square plane provided by the N,N′-(trimethyl-ethyleneammoniumthiosemicarbazanato)-4-(methylthiosemicarbazanato)butane-2,3-diimine ligand. The Ni—N1 and Ni—N2 bond distances of 1.860(3) and 1.846(3) Å, respectively, in 22 are the same as those observed in 21. While the Ni—S1 and Ni—S2 bond distances of 2.1642(11) and 2.1562(12) Å, respectively, in 22 are slightly longer than those observed in 21 (Table H1).

The observed longer Ni—S bonds in 22 are due to the iodide counter ion interacting with pendant NH resulting in elongation of the Ni—S bonds (FIG. 69). In addition, the Ni—S2 bond distance is longer than Ni—S1 due to the electron withdrawing trimethylammonium functional group. The main core framework in complex 22, ignoring the functional groups of the pendant amines, is rigorously planar with the largest deviation from the best fit plane of all 15 non-hydrogen atoms of ±0.0749 Å for Si and a standard deviation of ±0.0253 Å, which is nearly the same as in NiATSM. The distance between the calculated least squares planes (mean: 15 atoms) of the two stack molecules is 3.472 Å.

Compound 23 crystallizes as orange plates from slow diffusion of Et₂O into a MeCN/MeOH solution of 23 in the triclinic space group P-1. The Ni of 23 occupies an N₂S₂ square plane provided by the N,N′-bis(dimethylethylenediaminothiosemicarbazanato)butane-2,3-diimine ligand. The asymmetric unit of 23 consists of two, crystallographically distinct equivalents of NiATSDMEDA. Notably, the terminal dimethyl amino groups in the Nil molecule are oriented in the opposite way from the main N₂S₂ square plane, FIG. 70. While in the second Ni2 molecule they are oriented in the same direction, FIG. 71. The terminal dimethyl amino functional groups in the Ni2 molecule were found to be disordered. Modeled disorder did not help to significantly improve the crystal data quality. Therefore, bond distances and angles for compound 23 will not be discussed in details due to high standard deviation errors.

Green-brown plate crystals of 24 in the triclinic space group P-1 were obtained using vapor diffusion technique in an H-shaped tube by slow diffusion of Et₂O into a MeCN/MeOH solution of 24. The asymmetric unit of 24 consists of one equivalent of NiATSTMAEDA, FIG. 72. The Ni of 24 occupies an N₂S₂ square plane provided by the N,N′-bis(trimethylethylene-ammoniumthiosemicarbazanato)butane-2,3-diimine ligand. The Ni—N1 and Ni—N2 bond distances of 1.864(2) and 1.869(2) Å, respectively, in 24 are the same as those observed in 21 and 22. Nevertheless, the Ni—S1 and Ni—S2 bond distances of 2.1710(7) and 2.1715(7) Å, respectively, in 24 are longer than those observed in 21 and 22 (Table H1). The observed longer Ni—S bonds in 24 are due to the iodide counter ions interacting with the pendant NH and the symmetrical electron withdrawing trimethylammonium functional group resulting in elongation of the Ni—S bonds (FIG. 73). The main core framework in compound 24, ignoring the functional groups of the pendant amines, is rigorously planar with the largest deviation from the best fit plane of all 15 non-hydrogen atoms of ±0.0701 Å for S2 and a standard deviation of ±0.0375 Å, which is nearly the same as in 22.

The N2-Ni1-N1, N2-Ni1-S2, N1-Ni1-S1, and S2-Ni1-S1 bond angles in 21, 22, and 24 are the same (Table H1). The sum of the four bond angles around the Ni center in 21, 22, and 24 are 360.05(8), 360.01(10), and 360.04(6)°, respectively.

In the solid state crystal packing of 24, molecules are stacking into an ABAB arrangement along the b axis. The distance between the calculated least squares planes (mean: 15 atoms) of the two stack molecules is 3.485 Å.

FIG. 74 illustrates the unit cell (C2/c) of the cation of compound 22; iodide counter ion and hydrogens atoms are omitted for clarity. FIG. 75 illustrates the unit cell (P-1) of the cation of compound 24; iodide counter ions and hydrogens atoms are omitted for clarity.

Results and Discussion—Electrochemical Studies

The cyclic voltammograms of 21-24 were recorded in acetonitrile with tetrabuytlammonium hexafluorophosphate as a supporting electrolyte. The CV of 21-24 display two quasi-reversible events in the cathodic region from 0 to -2 V vs Fc⁺/Fc⁰ (FIGS. 76 and 77). In addition, all four complexes display an irreversible oxidation event between −1 and −1.3 V, which is only observed after scanning through the second reduction event (FIG. 78). The first event for 21 and 23, E_(1/2)=−1.73 and −1.72 V, respectively, is assigned to ligand centered reduction/oxidation, Ni(II)L/Ni(II)L⁻. The second event for 21 and 23 at −2.32 and −2.31 V, respectively, is assigned as metal-centered Ni^(II/I) couple. The CV of 21 and 23 in MeCN display similar quasi-reversible ligand-centered and metal-centered events as in the reported NiATSM at −1.73 and −2.31 V, respectively. However, the CV of alkylated compounds 22 and 24 display a significant shift, for both electrochemical events, in the anodic direction (FIGS. 76 and 77). Thus, the ligand-centered and metal-centered events in 22 are shifted by −80 and −90 mV, respectively, comparing to the non-alkylated compound 21 (Table H3). While in compound 24, both events are shifted by −140 and −170 mV, respectively, comparing to the non-alkylated compound 23. Therefore, as we anticipated, the double alkylation vs mono alkylation results in a doubling of the electrochemical shifts to a lower potential.

TABLE H3^(a) Compound NiL/NiL⁻ E_(1/2) (V) Ni^(II/I) E_(1/2) (V) 21 −1.73 (−80 mV) −2.32 (−90 mV) 22 −1.65 (−80 mV) −2.23 (−90 mV) 23 −1.72 (−140 mV) −2.31 (−170 mV) 24 −1.58 (−140 mV) −2.14 (−170 mV) ^(a)Data recorded on 0.3 mM of 21-24 in 0.1M Bu₄NPF₆ CH₃CN solution at a scan rate of 200 mV/s with potentials vs. Fc⁺/Fc⁰.

Results and Discussion—Electrocatalytic Hydrogen Evolution Studies

The performance of 21-24 as electrocatalysts for the hydrogen evolution reaction was evaluated in MeCN using as a proton source glacial acetic acid. The catalytic to peak current ratio (i_(cat)/i_(p)) increase linearly with the increase of the acid concentration. The value of i_(cat)/i_(p) saturates when the concentration of CH₃COOH reaches 100 mM (FIGS. 79 and 80).

The turnover frequencies (TOF) for 21-24 were determined using foot-of-the-wave analysis (FOWA) (COSTENTIN et al., “Turnover Numbers, Turnover Frequencies, and Overpotential in Molecular Catalysis of Electrochemical Reactions. Cyclic Voltammetry and Preparative-Scale Electrolysis” J. Am. Chem. Soc. (2012) Vol. 134, pp. 11235-11242). Thus, it was found that non-alkylated compounds 21 and 23 have a higher TOF of 2858 and 6296 s⁻¹, respectively, than the alkylated compounds 22 and 24 with a TOF of 1524 and 2416 s⁻¹, respectively. The same trend was observed for the overpotentials of 21-24 equal to 0.59, 0.66, 0.56, and 0.67 V, respectively. See Table H4. Compound 23 appears to be the most efficient catalyst due to the doubly equivalent basic tertiary amines on the pendant arms.

TABLE H4 Compound TOF (s⁻¹) η (V vs Fc⁺/Fc⁰) 21 2858 0.59 22 1524 0.66 23 6296 0.56 24 2416 0.67

Conclusions—Example Set H

It was demonstrated through electrochemical experiments that compounds 22 and 24 with the point-charges on the ligand framework shows a more anodic onset potential compared to non-alkylated parent compounds 21 and 23.

Cyclic voltammetry studies were performed on compounds 21-24 with addition of glacial acetic acid to see if the compounds are catalytically active. Compounds 21-24 were found to be effective catalysts for the hydrogen evolution reaction (HER).

Additional Embodiments

A1. A compound selected from a compound of Formula (I),

and

salts, optical isomers, geometric isomers, salts of isomers, and derivatives thereof, wherein

—R¹ is

or is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—X¹ is bivalent —(NH)—, —O—, —(CH₂)—, or —S—, which —(NH)— or —(CH₂)— can optionally be substituted with one or more (e.g., 0, 1, or 2) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—R² is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—R³ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—R⁴ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—R⁵ is

or is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—X² is bivalent —(NH)—, —O—, —(CH₂)—, or —S—, which —(NH)— or —(CH₂)— can optionally be substituted with one or more (e.g., 0, 1, or 2) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl; and

—R⁶ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl.

A2. The compound of embodiment A1, wherein

(a) R³ is the same as R⁴;

(b) R³ is the same as R⁴ and R¹ is the same as R⁵;

(c) R³ is the same as R⁴, X¹ is —(NH)—, X² is —(NH)—, and R² is the same as R⁶;

(d) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NH)—, and R² is —CH₃;

(e) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NH)—, and R² is —C₅H₆;

(f) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NH)—, and R² is —CH₂F₃;

(g) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NCH₃)—, and R² is —CH₃;

(h) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —O—, and R² is —CH₃;

(i) R³ is the same as R⁴ and R³ is methyl;

(j) R³ is the same as R⁴ and R³ is ethyl;

(k) R³ is methyl and R⁴ is phenyl;

(l) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —(N—CH(CH₃)₂)—, and R⁶ is —CH(CH₃)₂;

(m) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —(NH)—, and R⁶ is —C₅H₆;

(n) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —(NH)—, and R⁶ is —CH₂CF₃;

(o) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —O—, and R⁶ is —CH₃;

(p) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —O—, and R⁶ is —CH₂CH₃;

(q) (1) the limitations of (l) and (2) the limitations of (i), (j), or (k);

(r) (1) the limitations of (m) and (2) the limitations of (i), (j), or (k);

(s) (1) the limitations of (n) and (2) the limitations of (i), (j), or (k);

(t) (1) the limitations of (o) and (2) the limitations of (i), (j), or (k); or

(u) (1) the limitations of (p) and (2) the limitations of (i), (j), or (k).

A3. The compound of embodiment A1 or embodiment A2, wherein Formula (I) further comprises a solvent molecule coordinated with Formula (I).

A4. The compound of any of embodiments A1 to A3, wherein Formula (I) further comprises a solvent molecule coordinated with Formula (I) and the solvent molecule is selected from water, methanol, ethanol, propanol, acetonitrile, dimethylformamide, and acetone.

A5. The compound of any of embodiments A1 to A4, wherein the compound is

where R⁵ is pyridinyl, 1-methyl-imidazolyl, an N-containing heterocyclyl, or an N-containing heteroaryl.

A6. The compound of any of embodiments A1 to A5, wherein (a) X¹ is not —(NH)—, (b) R² is not methoxyphenyl or p-methoxyphenyl, or (c) both (a) and (b).

A7. The compound of any of embodiments A1 to A6, wherein (a) X² is not —(NH)—, (b) R⁶ is not methoxyphenyl or p-methoxyphenyl, or (c) both (a) and (b).

A8. The compound of any of embodiments A1 to A7, wherein (a) R³ is not methyl, (b) R⁴ is not methyl, or (c) both (a) and (b).

A9. The compound of any of embodiments A1 to A8, wherein the compound is not

A10. The compound of any of embodiments A1 to A9, wherein the compound is part of a homogenous solution, a homogenous aqueous solution, a heterogeneous solution, a heterogeneous aqueous solution, or a glassy carbon electrode.

A11. A homogenous solution, a homogenous aqueous solution, a heterogeneous solution, or a heterogeneous aqueous solution, each comprising the compound of any of embodiments A1 to A9.

A12. A glassy carbon electrode, a carbon paste (e.g., embedded with one or more of polynuclear catalysts, coordinated polymers, or metal-organic frameworks), covalent modified carbon (e.g., graphene), or non-covalent modified carbon (e.g., graphene), each comprising or reacted with the compound of any of embodiments A1 to A9.

A13. The carbon paste of embodiment A12, wherein the carbon paste comprises an extended structure motif (e.g., motif I, motif II, or motif III of scheme 11).

B1. A compound selected from Formula (II),

M.L (II) and

salts, optical isomers, geometric isomers, salts of isomers, and derivatives thereof, wherein

-M is Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, Co, Rh, Ti, V, Cr, Mn, or Fe (e.g., M is Cu⁺, Cut, Zn⁺, Co⁺, Ni⁺, Cd⁺, Mn⁺, Ru⁺, or Fe⁺; or M is Cu⁺, Cut, Zn⁺, Co⁺, Ni⁺, Cd⁺, Mn⁺, Ru⁺, or Fe⁺; or M is Cu⁺, Zn⁺, Ni⁺, Co⁺, Cd²⁺, Mn²⁺, or Fe²⁺; or M is Cu²⁺, Zn²⁺, Ni²⁺, or Co²⁺; or M is Cu²⁺, Zn²⁺, or Ni²⁺; or M is Cu²⁺, Zn²⁺, or Co²⁺; or M is Cu²⁺ or Zn²⁺) and

-L is selected from a thiosemicarbazone or a compound of Formula (I) of embodiments A1-A9.

B2. The compound of embodiment B1 wherein Formula (II) is a compound of Formula (II-A)

wherein

—R⁷ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—X³ is bivalent —(NH)—, —(N—CH(CH₃)₂)—, —(N—CH₂CH₃)—, —(N—CH₃)—, or —O—, which —(NH)—, —(N—CH(CH₃)₂)—, —(N—CH₂CH₃)—, or —(N—CH₃)— can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl, or , which —(N—CH(CH₃)₂)—, —(N—CH₂CH₃)—, or —(N—CH₃)— can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—R⁸ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—R⁹ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—X⁴ is bivalent —(NH)—, —(N—CH(CH₃)₂)—, —(N—CH₂CH₃)—, —(N—CH₃)—, or —O—, which —(NH)—, —(N—CH(CH₃)₂)—, —(N—CH₂CH₃)—, or —(N—CH₃)— can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl, or, which —(N—CH(CH₃)₂)—, —(N—CH₂CH₃)—, or —(N—CH₃)— can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl;

—R¹⁰ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen (e.g., F, Cl, Br, or I), aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl (e.g., C₁, C₂, C₃, C₄, C₅, C₆, or C₇ alkyl), C₂-C₇ alkenyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkenyl), C₂-C₇ alkynyl (e.g., C₂, C₃, C₄, C₅, C₆, or C₇ alkynyl), or C₁-C₆ alkoxy (C₁, C₂, C₃, C₄, C₅, or C₆ alkoxy), which aryl (e.g., benzene or pyrene), cycloalkyl, heterocyclyl, heteroaryl (e.g., pyridinyl and 1-methyl imidazolyl), C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more (e.g., 0, 1, 2, 3, 4, 5, or 6) of halogen (e.g., F, Cl, Br, or I), hydroxy (—OH), C₁-C₅ alkyl (e.g., C₁, C₂, C₃, C₄, or C₅ alkyl), C₁-C₄ alkoxy (C₁, C₂, C₃, or C₄ alkoxy), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl; and

-M is Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, Co, Rh, Ti, V, Cr, Mn, or Fe; or M is Cu²⁺, Cut, Zn²⁺, Co²⁺, Ni²⁺, Cd²⁺, Mn²⁺, Ru²⁺, or Fe²⁺; or M is Cu²⁺, Cut, Zn²⁺, Co²⁺, Cd²⁺, Mn²⁺, Ru²⁺, or Fe²⁺; or M is Cu²⁺, Zn²⁺, Co²⁺, Cd²⁺, Mn²⁺, or Fe²⁺; or M is Cu²⁺, Zn²⁺, Ni²⁺, or Co²⁺; or M is Cu²⁺, Zn²⁺, or Ni²⁺; or M is Cu²⁺, Zn²⁺, or Co²⁺; or M is Cu²⁺ or Zn²⁺.

B3. The compound of embodiment B1 or embodiment B2, wherein

(a) R³ is the same as R⁴;

(b) R³ is the same as R⁴ and R¹ is the same as R⁵;

(c) R³ is the same as R⁴, X¹ is —(NH)—, X² is —(NH)—, and R² is the same as R⁶;

(d) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NH)—, and R² is —CH₃;

(e) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NH)—, and R² is —C₅H₆;

(f) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NH)—, and R² is —CH₂F₃;

(g) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NCH₃)—, and R² is —CH₃;

(h) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —O—, and R² is —CH₃;

(i) R³ is the same as R⁴ and R³ is methyl;

(j) R³ is the same as R⁴ and R³ is ethyl;

(k) R³ is methyl and R⁴ is phenyl;

(l) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —(N—CH(CH₃)₂)—, and R⁶ is —CH(CH₃)₂;

(m) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —(NH)—, and R⁶ is —C₅H₆;

(n) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —(NH)—, and R⁶ is —CH₂CF₃;

(o) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —O—, and R⁶ is —CH₃;

(p) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —O—, and R⁶ is —CH₂CH₃;

(q) (1) the limitations of (l) and (2) the limitations of (i), (j), or (k);

(r) (1) the limitations of (m) and (2) the limitations of (i), (j), or (k);

(s) (1) the limitations of (n) and (2) the limitations of (i), (j), or (k);

(t) (1) the limitations of (o) and (2) the limitations of (i), (j), or (k); or

(u) (1) the limitations of (p) and (2) the limitations of (i), (j), or (k).

B4. The compound of any of embodiments B1 to B3, wherein

(a) R⁸ is the same as R⁹;

(b) R⁸ is the same as R⁹ and X³—R⁷ is the same as X⁴—R¹⁰;

(c) R⁸ is the same as R⁹, X³ is —(NH)—, X⁴ is —(NH)—, and R⁷ is the same as R¹⁰;

(d) R⁸ is the same as R⁹, R⁸ is methyl, X³—R⁷ is the same as X⁴—R¹⁰, X³ is —(NH)—, and R⁷ is —CH₃;

(e) R⁸ is the same as R⁹, R⁸ is methyl, X³—R⁷ is the same as X⁴—R¹⁰, X³ is —(NH)—, and R⁷ is C₅H₆;

(f) R⁸ is the same as R⁹, R⁸ is methyl, X³—R⁷ is the same as X⁴—R¹⁰, X³ is —(NH)—, and R⁷ is —CH₂F₃;

(g) R⁸ is the same as R⁹, R⁸ is methyl, X³—R⁷ is the same as X⁴—R¹⁰, X³ is —(NCH₃)—, and R⁷ is —CH₃;

(h) R⁸ is the same as R⁹, R⁸ is methyl, X³—R⁷ is the same as X⁴—R¹⁰, X³ is —O—, and R⁷ is —CH₃;

(i) R⁸ is the same as R⁹ and R⁸ is methyl;

(j) R⁸ is the same as R⁹ and R⁸ is ethyl;

(k) R⁸ is methyl and R⁹ is phenyl;

(l) X³ is —(NH)—, R⁷ is —CH₃, X⁴ is —(N—CH(CH₃)₂)—, and R¹⁰ is —CH(CH₃)₂;

(m) X³ is —(NH)—, R⁷ is —CH₃, X⁴ is —(NH)—, and R¹⁰ is —C₅H₆;

(n) X³ is —(NH)—, R⁷ is —CH₃, X⁴ is —(NH)—, and R¹⁰ is —CH₂CF₃;

(o) X³ is —(NH)—, R⁷ is —CH₃, X⁴ is —O—, and R¹⁰ is —CH₃;

(p) X³ is —(NH)—, R⁷ is —CH₃, X⁴ is —O—, and R¹⁰ is —CH₂CH₃;

(q) (1) the limitations of (l) and (2) the limitations of (i), (j), or (k);

(r) (1) the limitations of (m) and (2) the limitations of (i), (j), or (k);

(s) (1) the limitations of (n) and (2) the limitations of (i), (j), or (k);

(t) (1) the limitations of (o) and (2) the limitations of (i), (j), or (k); or

(u) (1) the limitations of (p) and (2) the limitations of (i), (j), or (k).

B5. The compound of any of embodiments B1 to B4, wherein the M is Zn²⁺, Co²⁺, Ni²⁺, or Cu²⁺ (e.g., M is Zn²⁺, Co²⁺, or Cu²⁺).

B6. The compound of any of embodiments B1 to B5, wherein Formula (II) further comprises a solvent molecule coordinated with Formula (II).

B7. The compound of any of embodiments B1 to B6, wherein Formula (II) further comprises a solvent molecule coordinated with Formula (II) and the solvent molecule is selected from water, ethanol, propanol, acetonitrile, dimethylformamide, and acetone.

B8. The compound of any of embodiments B1 to B7, wherein the compound is

where M is Zn, Co, Ni, or Cu (e.g., M is Zn, Co, or Cu; or M is Zn, Ni, or Cu; or M is Zn or Cu) and R⁵ is pyridinyl, 1-methyl-imidazolyl, an N-containing heterocyclyl, or an N-containing heteroaryl.

B9. The compound of any of embodiments B1 to B8, wherein (a) X¹ is not —(NH)—, (b) R² is not methoxyphenyl or p-methoxyphenyl, or (c) both (a) and (b).

B10. The compound of any of embodiments B1 to B9, wherein (a) X² is not —(NH)—, (b) R⁶ is not methoxyphenyl or p-methoxyphenyl, or (c) both (a) and (b).

B11. The compound of any of embodiments B1 to B10, wherein (a) R³ is not methyl, (b) R⁴ is not methyl, or (c) both (a) and (b).

B12. The compound of any of embodiments B1 to B11, wherein (a) X³ is not —(NH)—, (b) R⁷ is not methoxyphenyl or p-methoxyphenyl, or (c) both (a) and (b).

B13. The compound of any of embodiments B1 to B12, wherein (a) X⁴ is not —(NH)—, (b) R¹⁰ is not methoxyphenyl or p-methoxyphenyl, or (c) both (a) and (b).

B14. The compound of any of embodiments B1 to B13, wherein (a) R⁸ is not methyl, (b) R⁹ is not methyl, or (c) both (a) and (b).

B15. The compound of any of embodiments B1 to B14, wherein the compound is not

B16. The compound of any of embodiments B1 to B15, wherein the compound is part of a homogenous solution, a homogenous aqueous solution, a heterogeneous solution, or a heterogeneous aqueous solution.

B17. A homogenous solution, a homogenous aqueous solution, a heterogeneous solution, or a heterogeneous aqueous solution, each comprising the compound of any of embodiments B1 to B16.

B18. A glassy carbon electrode, a carbon paste (e.g., embedded with one or more of polynuclear catalysts, coordinated polymers, or metal-organic frameworks), covalent modified carbon (e.g., graphene), or non-covalent modified carbon (e.g., graphene), each comprising or reacted with the compound of any of embodiments B1 to B17.

B19. The carbon paste of embodiment B18, wherein the carbon paste comprises an extended structure motif (e.g., motif I, motif II, or motif III of scheme 11).

C1. A catalyst (e.g., an electrocatalyst) comprising a composition comprising a compound of Formula (I) (e.g., embodiments A1-A13), a compound of Formula (II) (e.g., embodiments B1-B19), or both.

D1. An anode comprising a composition comprising a compound of Formula (I) (e.g., embodiments A1-A13), a compound of Formula (II) (e.g., embodiments B1-B19), or both.

E1. A cathode comprising a composition comprising a compound of Formula (I) (e.g., embodiments A1-A13), a compound of Formula (II) (e.g., embodiments B1-B19), or both.

F1. An electrochemical cell comprising a composition comprising a compound of Formula (I) (e.g., embodiments A1-A13), a compound of Formula (II) (e.g., embodiments B1-B19), or both.

F2. The electrochemical cell of embodiment Fl, wherein the cathode of the electrochemical cell comprises the composition.

G1. A fuel cell comprising a composition comprising a compound of Formula (I) (e.g., embodiments A1-A13), a compound of Formula (II) (e.g., embodiments B1-B19), or both.

G2. The fuel cell of embodiment G1, wherein the anode of the fuel cell comprises the first composition.

H1. A method for producing H2 comprising contacting, in an electrochemical cell, a first composition comprising a compound of Formula (I) (e.g., embodiments A1-A13), a compound of Formula (II) (e.g., embodiments B1-B19), or both with a second composition comprising water.

H2. The method of embodiment H1, wherein the cathode of the electrochemical cell comprises the first composition.

H3. The method of H1 or H2, wherein the TOF is from about 20 s⁻¹ to about 100,000 s⁻¹, about 100 s⁻¹ to about 100,000 s⁻¹, from about 500 s⁻¹ to about 100,000 s⁻¹, from about 500 s⁻¹ to about 50,000 s⁻¹, from about 500 s⁻¹ to about 20,000 s⁻¹, about 20 s⁻¹, about 100 s⁻¹, about 500 s⁻¹, about 1000 s⁻¹, about 5000 s⁻¹, about 10000 s⁻¹, about 12000 s⁻¹, about 16000 s⁻¹, about 20000 s⁻¹, about 50000 s⁻¹, or about 100,000 s⁻¹.

H4. A method for oxidizing an aldehyde or an alcohol comprising contacting, in an electrochemical cell, a composition comprising a compound of Formula (I) (e.g., embodiments A1-A13), a compound of Formula (II) (e.g., embodiments B1-B19), or both.

H5. The method of any of H1-H4, wherein the overpotential is greater than about 0 V, not less than about 0.1 V, not more than about 0.1 V, not more than 0.5 V, not more than 1 V, not more than 10 V, not more than 100 V, from about 0 V to about 2000 V, from about 0 V to about 1000 V, from about 0 V to about 750 V, from about 0 V to about 300 V, from about 0 V to about 350 V, from about 0 V to about 200 V, from about 0 V to about 100 V, from about 0 V to about 20 V, from about 0 V to about 10 V, from about 0 V to about 5 V, from about 0 V to about 2 V, from about 0 V to about 1 V, from about 0.1 V to about 2000 V, from about 0.1 V to about 1000 V, from about 0.1 V to about 750 V, from about 0.1 V to about 300 V, from about 0.1 V to about 350 V, from about 0.1 V to about 200 V, from about 0.1 V to about 100 V, from about 0.1 V to about 20 V, from about 0.1 V to about 10 V, from about 0.1 V to about 5 V, from about 0.1 V to about 2 V, from about 0.1 V to about 1 V, about 0.1 V, about 0.5 V, about 1 V, about 5 V, about 10 V, about 100 V, about 250 V, about 350 V, about 400 V, about 500 V, or about 1000 V.

I1. A method for producing electricity comprising contacting, in a fuel cell, a first composition comprising a compound of Formula (I) (e.g., embodiments A1-A13), a compound of Formula (II) (e.g., embodiments B1-B19), or both with a second composition comprising H2.

I2. The method of embodiment I1, wherein the anode of the fuel cell comprises the first composition.

I3. The method of I1 or I2, wherein the TOF is from about 1 s⁻¹ to about 1000 s⁻¹, from about 5 s⁻¹ to about 1000 s⁻¹, from about 5 s⁻¹ to about 500 s⁻¹, from about 5 s⁻¹ to about 200 s⁻¹, about 1 s⁻¹, about 5 s⁻¹, about 10 s⁻¹, about 32 s⁻¹, about 50 s⁻¹, about 76 s⁻¹, about 100 s⁻¹, about 120 s⁻¹, about 200 s⁻¹, about 300 s⁻¹, about 500 s⁻¹, or about 1000 s⁻¹.

I4. The method of any of I1-I3, wherein the overpotential is greater than about 0 V, not less than about 0.1 V, not more than about 0.1 V, not more than 0.5 V, not more than 1 V, not more than 10 V, not more than 100 V, from about 0 V to about 2000 V, from about 0 V to about 1000 V, from about 0 V to about 750 V, from about 0 V to about 300 V, from about 0 V to about 350 V, from about 0 V to about 200 V, from about 0 V to about 100 V, from about 0 V to about 20 V, from about 0 V to about 10 V, from about 0 V to about 5 V, from about 0 V to about 2 V, from about 0 V to about 1 V, from about 0.1 V to about 2000 V, from about 0.1 V to about 1000 V, from about 0.1 V to about 750 V, from about 0.1 V to about 300 V, from about 0.1 V to about 350 V, from about 0.1 V to about 200 V, from about 0.1 V to about 100 V, from about 0.1 V to about 20 V, from about 0.1 V to about 10 V, from about 0.1 V to about 5 V, from about 0.1 V to about 2 V, from about 0.1 V to about 1 V, about 0.1 V, about 0.5 V, about 1 V, about 5 V, about 10 V, about 100 V, about 250 V, about 350 V, about 400 V, about 500 V, or about 1000 V.

J1. A method for preparing a compound of Formula (I) (e.g., embodiments A1-A13) comprising any suitable method.

J2. The method of J1, wherein the compound of Formula (I) is prepared comprising

(a) reacting a compound of Formula (III)

with a compound of Formula (IV)

(b) reacting a compound of Formula (V)

with a compound of Formula (VI)

and

(c) recovering the compound of Formula (I),

wherein R¹, R³, R⁴, and R⁵ are defined as in embodiments A1-A13.

K1. A method for preparing a compound of Formula (II) (e.g., embodiments B1-B19) comprising any suitable method.

K2. The method of K1, wherein the compound of Formula (II) is prepared comprising

(a) reacting a compound of Formula (I) (e.g., embodiments A1-A13) with M; and

(b) recovering the compound of Formula (II),

wherein M is defined as in embodiments B1-B19.

L1. A method for preparing a catalyst (e.g., an electrocatalyst) comprising a composition comprising a compound of Formula (I) (e.g., embodiments A1-A13), a compound of Formula (II) (e.g., embodiments B1-B19), or both, comprising any suitable method.

M1. A method for preparing an anode comprising a composition comprising a compound of Formula (I) (e.g., embodiments A1-A13), a compound of Formula (II) (e.g., embodiments B1-B19), or both, comprising any suitable method.

N1. A method for preparing a cathode comprising a composition comprising a compound of Formula (I) (e.g., embodiments A1-A13), a compound of Formula (II) (e.g., embodiments B1-B19), or both, comprising any suitable method.

The headings used in the disclosure are not meant to suggest that all disclosure relating to the heading is found within the section that starts with that heading. Disclosure for any subject may be found throughout the specification.

It is noted that terms like “preferably,” “commonly,” and “typically” are not used herein to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention.

As used in the disclosure, “a” or “an” means one or more than one, unless otherwise specified. As used in the claims, when used in conjunction with the word “comprising” the words “a” or “an” means one or more than one, unless otherwise specified. As used in the disclosure or claims, “another” means at least a second or more, unless otherwise specified. As used in the disclosure, the phrases “such as”, “for example”, and “e.g.” mean “for example, but not limited to” in that the list following the term (“such as”, “for example”, or “e.g.”) provides some examples but the list is not necessarily a fully inclusive list. The word “comprising” means that the items following the word “comprising” may include additional unrecited elements or steps; that is, “comprising” does not exclude additional unrecited steps or elements.

Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in this specification and claims are approximations that can vary depending upon the desired properties or functions sought to be obtained by the presently-disclosed subject matter.

As used herein, the term “about,” when referring to a value or to an amount of mass, weight, time, volume, concentration or percentage is meant to encompass variations of in some embodiments ±20%, in some embodiments ±10%, in some embodiments ±5%, in some embodiments ±1%, in some embodiments ±0.5%, and in some embodiments ±0.1% from the specified amount, as such variations are appropriate to perform the disclosed method.

Detailed descriptions of one or more embodiments are provided herein. It is to be understood, however, that the present invention may be embodied in various forms. Therefore, specific details disclosed herein (even if designated as preferred or advantageous) are not to be interpreted as limiting, but rather are to be used as an illustrative basis for the claims and as a representative basis for teaching one skilled in the art to employ the present invention in any appropriate manner. Indeed, various modifications of the invention in addition to those described herein will become apparent to those skilled in the art from the foregoing description and the accompanying figures. Such modifications are intended to fall within the scope of the appended claims. 

1. A compound selected from a compound of Formula (I),

and salts, optical isomers, geometric isomers, salts of isomers, and derivatives thereof, wherein —R¹ is

or is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen, aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy, which aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl; —X¹ is bivalent —(NH)—, —O—, —(CH₂)—, or —S—, which —(NH)— or —(CH₂)— can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl; —R² is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen, aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy, which aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, amine (—NH₂), —NR_(a)R_(b), —N⁽⁺⁾R_(a)R_(b)R_(c), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl; —R³ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen, aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy, which aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl; —R⁴ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen, aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy, which aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl; —R⁵ is

or is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen, aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy, which aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl; —X² is bivalent —(NH)—, —O—, —(CH₂)—, or —S—, which —(NH)— or —(CH₂)— can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl; —R⁶ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen, aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy, which aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, amine (—NH₂), —NR_(a)R_(b), —N⁽⁺⁾R_(a)R_(b)R_(c), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl; and —R_(a), R_(b), and R_(c) are each independently selected from C₁-C₅ alkyl.
 2. The compound of claim 1, wherein (a) R³ is the same as R⁴; (b) R³ is the same as R⁴ and R¹ is the same as R⁵; (c) R³ is the same as R⁴, X¹ is —(NH)—, X² is —(NH)—, and R² is the same as R⁶; (d) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NH)—, and R² is —CH₃; (e) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NH)—, and R² is —C₅H₆; (f) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NH)—, and R² is —CH₂F₃; (g) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —(NCH₃)—, and R² is —CH₃; (h) R³ is the same as R⁴, R³ is methyl, R¹ is the same as R⁵, R¹ is

X¹ is —O—, and R² is —CH₃; (i) R³ is the same as R⁴ and R³ is methyl; (j) R³ is the same as R⁴ and R³ is ethyl; (k) R³ is methyl and R⁴ is phenyl; (l) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —(N—CH(CH₃)₂)—, and R⁶ is —CH(CH₃)₂; (m) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —(NH)—, and R⁶ is —C₅H₆; (n) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —(NH)—, and R⁶ is —CH₂CF₃; (o) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —O—, and R⁶ is —CH₃; (p) R¹ is

X¹ is —(NH)—, R² is —CH₃, R⁵ is

X² is —O—, and R⁶ is —CH₂CH₃; (q) (1) the limitations of (l) and (2) the limitations of (i), (j), or (k); (r) (1) the limitations of (m) and (2) the limitations of (i), (j), or (k); (s) (1) the limitations of (n) and (2) the limitations of (i), (j), or (k); (t) (1) the limitations of (o) and (2) the limitations of (i), (j), or (k); or (u) (1) the limitations of (p) and (2) the limitations of (i), (j), or (k).
 3. The compound of claim 1, wherein Formula (I) further comprises a solvent molecule coordinated with Formula (I).
 4. The compound of claim 1, wherein one or both of R² or R⁶ is (a) C₁-C₇ alkyl substituted with —NR_(a)R_(b), (b) C₁-C₇ alkyl substituted with —N⁽⁺⁾R_(a)R_(b)R_(c), (c) substituted or unsubstituted benzo crown ether, (d) phenyl substituted with a carboxy, (e) substituted or unsubstituted pyrrolyl, (f) substituted or unsubstituted pyridyl, or (g) substituted or unsubstituted imidazolyl.
 5. The compound of claim 1, wherein the compound is

where R⁵ is pyridinyl, 1-methyl-imidazolyl, an N-containing heterocyclyl, or an N-containing heteroaryl.
 6. The compound of claim 1, wherein (a) X¹ is not —(NH)—, (b) R² is not methoxyphenyl or p-methoxyphenyl, or (c) both (a) and (b).
 7. The compound of claim 1, wherein (a) X² is not —(NH)—, (b) R⁶ is not methoxyphenyl or p-methoxyphenyl, or (c) both (a) and (b).
 8. The compound of claim 1, wherein (a) R³ is not methyl, (b) R⁴ is not methyl, or (c) both (a) and (b).
 9. The compound of claim 1, wherein the compound is not


10. A compound selected from a compound of Formula (II), M.L (II) and salts, optical isomers, geometric isomers, salts of isomers, and derivatives thereof, wherein M is Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, Co, Rh, Ti, V, Cr, Mn, or Fe; and L is a compound of Formula (I) of any of claims 1-9.
 11. The compound of claim 10, wherein Formula (II) is a compound of Formula (II-A)

wherein —R⁷ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen, aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy, which aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, amine (—NH₂), —NR_(a)R_(b), —N⁽⁺⁾R_(a)R_(b)R_(c), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl; —X³ is bivalent —(NH)—, —(N—CH(CH₃)₂)—, —(N—CH₂CH₃)—, —(N—CH₃)—, or —O—, which —(NH)—, —(N—CH(CH₃)₂)—, —(N—CH₂CH₃)—, or —(N—CH₃)— can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl; —R⁸ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen, aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy, which aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl; —R⁹ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen, aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy, which aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl; —X⁴ is bivalent —(NH)—, —(N—CH(CH₃)₂)—, —(N—CH₂CH₃)—, —(N—CH₃)—, or —O—, which —(NH)—, —(N—CH(CH₃)₂)—, —(N—CH₂CH₃)—, or —(N—CH₃)— can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl; —R¹⁰ is a monovalent H, methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), sulfo (—SO₃H), halogen, aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy, which aryl, cycloalkyl, heterocyclyl, heteroaryl, C₁-C₇ alkyl, C₂-C₇ alkenyl, C₂-C₇ alkynyl, or C₁-C₆ alkoxy can optionally be substituted with one or more of halogen, hydroxy (—OH), C₁-C₅ alkyl, C₁-C₄ alkoxy, amine (—NH₂), —NR_(a)R_(b), —N⁽⁺⁾R_(a)R_(b)R_(c), methanoly (—COH), carboxy (—CO₂H), nitro (—NO₂), cyano (—CN), ethynyl (—CCH), sulfo (—SO₃H), methyl, or ethyl; —R_(a), R_(b), and R_(c) are each independently selected from C₁-C₅ alkyl; and -M is Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, Co, Rh, Ti, V, Cr, Mn, or Fe.
 12. The compound of claim 10, wherein (a) R⁸ is the same as R⁹; (b) R⁸ is the same as R⁹ and X³—R⁷ is the same as X⁴—R¹⁰; (c) R⁸ is the same as R⁹, X³ is —(NH)—, X⁴ is —(NH)—, and R⁷ is the same as R¹⁰; (d) R⁸ is the same as R⁹, R⁸ is methyl, X³—R⁷ is the same as X⁴—R¹⁰, X³ is —(NH)—, and R⁷ is —CH₃; (e) R⁸ is the same as R⁹, R⁸ is methyl, X³—R⁷ is the same as X⁴—R¹⁰, X³ is —(NH)—, and R⁷ is —C₅H₆; (f) R⁸ is the same as R⁹, R⁸ is methyl, X³—R⁷ is the same as X⁴—R¹⁰, X³ is —(NH)—, and R⁷ is —CH₂F₃; (g) R⁸ is the same as R⁹, R⁸ is methyl, X³—R⁷ is the same as X⁴—R¹⁰, X₃ is —(NCH₃)—, and R⁷ is —CH₃; (h) R⁸ is the same as R⁹, R⁸ is methyl, X³—R⁷ is the same as X⁴—R¹⁰, X³ is —O—, and R⁷ is —CH₃; (i) R⁸ is the same as R⁹ and R⁸ is methyl; (j) R⁸ is the same as R⁹ and R⁸ is ethyl; (k) R⁸ is methyl and R⁹ is phenyl; (l) X³ is —(NH)—, R⁷ is —CH₃, X⁴ is —(N—CH(CH₃)₂)—, and R¹⁰ is —CH(CH₃)₂; (m) X³ is —(NH)—, R⁷ is —CH₃, X⁴ is —(NH)—, and R¹⁰ is —C₅H₆; (n) X³ is —(NH)—, R⁷ is —CH₃, X⁴ is —(NH)—, and R¹⁰ is —CH₂CF₃; (o) X³ is —(NH)—, R⁷ is —CH₃, X⁴ is —O—, and R¹⁰ is —CH₃; (p) X³ is —(NH)—, R⁷ is —CH₃, X⁴ is —O—, and R¹⁰ is —CH₂CH₃; (q) (1) the limitations of (l) and (2) the limitations of (i), (j), or (k); (r) (1) the limitations of (m) and (2) the limitations of (i), (j), or (k); (s) (1) the limitations of (n) and (2) the limitations of (i), (j), or (k); (t) (1) the limitations of (o) and (2) the limitations of (i), (j), or (k); or (u) (1) the limitations of (p) and (2) the limitations of (i), (j), or (k).
 13. The compound of claim 10, wherein the M is Zn²⁺, Co²⁺, Ni²⁺, or Cu²⁺.
 14. The compound of claim 10, wherein Formula (II) further comprises a solvent molecule coordinated with Formula (II).
 15. The compound of claim 10, wherein one or both of R⁷ or R¹⁰ is (a) C₁-C₇ alkyl substituted with —NR_(a)R_(b), (b) C₁-C₇ alkyl substituted with —N⁽⁺⁾R_(a)R_(b)R_(c), (c) substituted or unsubstituted benzo crown ether, (d) phenyl substituted with a carboxy, (e) substituted or unsubstituted pyrrolyl, (f) substituted or unsubstituted pyridyl, or (g) substituted or unsubstituted imidazolyl.
 16. The compound of claim 10, wherein the compound is

where M is Zn, Co, Ni, or Cu and R⁵ is pyridinyl, 1-methyl-imidazolyl, an N-containing heterocyclyl, or an N-containing heteroaryl.
 17. The compound of claim 10, wherein (a) X³ is not —(NH)—, (b) R⁷ is not methoxyphenyl or p-methoxyphenyl, or (c) both (a) and (b).
 18. The compound of claim 10, wherein (a) X⁴ is not —(NH)—, (b) R¹⁰ is not methoxyphenyl or p-methoxyphenyl, or (c) both (a) and (b).
 19. The compound of claim 10, wherein (a) R⁸ is not methyl, (b) R⁹ is not methyl, or (c) both (a) and (b).
 20. The compound of claim 10, wherein the compound is not


21. A catalyst or an electrocatalyst comprising a composition comprising a compound of Formula (I), a compound of Formula (II), or both.
 22. An electrochemical cell comprising a composition comprising a compound of Formula (I), a compound of Formula (II), or both.
 23. The electrochemical cell of claim 22, wherein the cathode of the electrochemical cell comprises the composition.
 24. A method for producing H₂ comprising contacting, in an electrochemical cell, a first composition comprising a compound of Formula (I), a compound of Formula (II), or both, with a second composition comprising water.
 25. The method of claim 24, wherein the cathode of the electrochemical cell comprises the first composition.
 26. The method of claim 24, wherein the TOF is from about 20 s⁻¹ to about 100,000 s⁻¹, about 100 s⁻¹ to about 100,000 s⁻¹, from about 500 s⁻¹ to about 100,000 s⁻¹, from about 500 s⁻¹ to about 50,000 s⁻¹, from about 500 s⁻¹ to about 20,000 s⁻¹, about 20 s⁻¹, about 100 s⁻¹, about 500 s⁻¹, about 1000 s⁻¹, about 5000 s⁻¹, about 10000 s⁻¹, about 12000 s⁻¹, about 16000 s⁻¹, about 20000 s⁻¹, about 50000 s⁻¹, or about 100,000 s⁻¹. 